From 2fb942e4230b97c120e3efe31481847a2223c848 Mon Sep 17 00:00:00 2001
From: Erik Christensen <40830816+erikc5000@users.noreply.github.com>
Date: Wed, 5 Aug 2020 10:56:00 -0400
Subject: [PATCH] Re-work clock implementation (#118)

* Re-work the clock implementation on JVM

* Re-work new clock implementation to handle Darwin

* Minor clean up
---
 .../kotlin/io/islandtime/Instant.kt           |   5 +
 .../kotlin/io/islandtime/clock/Clock.kt       | 287 +++++++-----------
 .../kotlin/io/islandtime/clock/FixedClock.kt  | 159 ++++++++++
 .../kotlin/io/islandtime/clock/Now.kt         | 116 +++----
 .../kotlin/io/islandtime/clock/SystemClock.kt |  42 +++
 .../io/islandtime/clock/internal/NowImpl.kt   |  11 +
 .../clock/internal/SystemClockImpl.kt         |   6 +
 .../io/islandtime/internal/Deprecation.kt     |   3 +
 .../kotlin/io/islandtime/internal/Instants.kt |  26 ++
 .../kotlin/io/islandtime/internal/Platform.kt |   5 -
 .../io/islandtime/internal/PlatformImpl.kt    |  10 +
 .../io/islandtime/zone/TimeZoneRules.kt       |  32 +-
 .../kotlin/io/islandtime/clock/ClockTest.kt   |  12 +-
 .../kotlin/io/islandtime/clock/NowTest.kt     |  26 +-
 .../kotlin/io/islandtime/PlatformInstant.kt   |   5 +
 .../islandtime/clock/PlatformSystemClock.kt   |  20 --
 .../io/islandtime/clock/internal/NowImpl.kt   |  41 +++
 .../clock/internal/SystemClockImpl.kt         |  38 +++
 .../kotlin/io/islandtime/internal/Platform.kt |   8 -
 .../io/islandtime/internal/PlatformImpl.kt    |  15 +
 .../io/islandtime/zone/TimeZoneRules.kt       |  13 +-
 .../kotlin/io/islandtime/PlatformInstant.kt   |   3 +
 .../islandtime/clock/PlatformSystemClock.kt   |   7 -
 .../islandtime/clock/internal/Conversions.kt  |  35 +++
 .../io/islandtime/clock/internal/NowImpl.kt   |  28 ++
 .../clock/internal/SystemClockImpl.kt         |  21 ++
 .../kotlin/io/islandtime/clock/jvm/Now.kt     |  64 ++++
 .../kotlin/io/islandtime/internal/Platform.kt |  18 --
 .../io/islandtime/internal/PlatformImpl.kt    |  34 +++
 .../kotlin/io/islandtime/jvm/Conversions.kt   |  21 +-
 .../io/islandtime/zone/TimeZoneRules.kt       |  19 +-
 .../io/islandtime/clock/jvm/JvmNowTest.kt     | 103 +++++++
 .../{PlatformTest.kt => PlatformImplTest.kt}  |  12 +-
 .../io/islandtime/jvm/ConversionsTest.kt      |  24 ++
 34 files changed, 900 insertions(+), 369 deletions(-)
 create mode 100644 core/src/commonMain/kotlin/io/islandtime/clock/FixedClock.kt
 create mode 100644 core/src/commonMain/kotlin/io/islandtime/clock/SystemClock.kt
 create mode 100644 core/src/commonMain/kotlin/io/islandtime/clock/internal/NowImpl.kt
 create mode 100644 core/src/commonMain/kotlin/io/islandtime/clock/internal/SystemClockImpl.kt
 create mode 100644 core/src/commonMain/kotlin/io/islandtime/internal/Deprecation.kt
 create mode 100644 core/src/commonMain/kotlin/io/islandtime/internal/Instants.kt
 delete mode 100644 core/src/commonMain/kotlin/io/islandtime/internal/Platform.kt
 create mode 100644 core/src/commonMain/kotlin/io/islandtime/internal/PlatformImpl.kt
 create mode 100644 core/src/darwinMain/kotlin/io/islandtime/PlatformInstant.kt
 delete mode 100644 core/src/darwinMain/kotlin/io/islandtime/clock/PlatformSystemClock.kt
 create mode 100644 core/src/darwinMain/kotlin/io/islandtime/clock/internal/NowImpl.kt
 create mode 100644 core/src/darwinMain/kotlin/io/islandtime/clock/internal/SystemClockImpl.kt
 delete mode 100644 core/src/darwinMain/kotlin/io/islandtime/internal/Platform.kt
 create mode 100644 core/src/darwinMain/kotlin/io/islandtime/internal/PlatformImpl.kt
 create mode 100644 core/src/jvmMain/kotlin/io/islandtime/PlatformInstant.kt
 delete mode 100644 core/src/jvmMain/kotlin/io/islandtime/clock/PlatformSystemClock.kt
 create mode 100644 core/src/jvmMain/kotlin/io/islandtime/clock/internal/Conversions.kt
 create mode 100644 core/src/jvmMain/kotlin/io/islandtime/clock/internal/NowImpl.kt
 create mode 100644 core/src/jvmMain/kotlin/io/islandtime/clock/internal/SystemClockImpl.kt
 create mode 100644 core/src/jvmMain/kotlin/io/islandtime/clock/jvm/Now.kt
 delete mode 100644 core/src/jvmMain/kotlin/io/islandtime/internal/Platform.kt
 create mode 100644 core/src/jvmMain/kotlin/io/islandtime/internal/PlatformImpl.kt
 create mode 100644 core/src/jvmTest/kotlin/io/islandtime/clock/jvm/JvmNowTest.kt
 rename core/src/jvmTest/kotlin/io/islandtime/internal/{PlatformTest.kt => PlatformImplTest.kt} (79%)

diff --git a/core/src/commonMain/kotlin/io/islandtime/Instant.kt b/core/src/commonMain/kotlin/io/islandtime/Instant.kt
index 892eeb0bb..6b86e3113 100644
--- a/core/src/commonMain/kotlin/io/islandtime/Instant.kt
+++ b/core/src/commonMain/kotlin/io/islandtime/Instant.kt
@@ -10,6 +10,11 @@ import io.islandtime.measures.internal.plusWithOverflow
 import io.islandtime.parser.*
 import io.islandtime.ranges.InstantInterval
 
+/**
+ * A platform-specific representation of an instant in time.
+ */
+expect class PlatformInstant
+
 /**
  * An instant in time with nanosecond precision.
  */
diff --git a/core/src/commonMain/kotlin/io/islandtime/clock/Clock.kt b/core/src/commonMain/kotlin/io/islandtime/clock/Clock.kt
index 80cf54e54..3e94f6b98 100644
--- a/core/src/commonMain/kotlin/io/islandtime/clock/Clock.kt
+++ b/core/src/commonMain/kotlin/io/islandtime/clock/Clock.kt
@@ -1,11 +1,21 @@
+@file:Suppress("FunctionName", "UNUSED_PARAMETER")
+
 package io.islandtime.clock
 
 import io.islandtime.Instant
+import io.islandtime.PlatformInstant
 import io.islandtime.TimeZone
+import io.islandtime.internal.deprecatedToError
 import io.islandtime.measures.*
 
 /**
  * A time source.
+ *
+ * For an implementation that uses the system's clock, see [SystemClock]. [FixedClock] is also available for testing
+ * purposes.
+ *
+ * @see SystemClock
+ * @see FixedClock
  */
 interface Clock {
     /**
@@ -14,190 +24,107 @@ interface Clock {
     val zone: TimeZone
 
     /**
-     * Get the current number of milliseconds since the Unix epoch of 1970-01-01T00:00 in UTC.
+     * Reads the current number of milliseconds that have elapsed since the Unix epoch of `1970-01-01T00:00` in UTC.
      */
-    fun read(): LongMilliseconds
+    fun readMilliseconds(): LongMilliseconds
 
     /**
-     * Get the current [Instant].
+     * Reads the current [Instant].
      */
-    fun instant(): Instant = Instant(read())
-}
-
-/**
- * A clock that provides the time from the current system.
- *
- * The time zone is treated as an immutable property of the clock, set when it is created. If you wish to follow
- * changes to the system clock's configured time zone, you must create a new [SystemClock] in response to any time zone
- * changes.
- *
- * @constructor Create a [SystemClock] with a specific time zone, defaulting to the system's current zone.
- * @see currentZone
- */
-class SystemClock(override val zone: TimeZone = TimeZone.systemDefault()) : Clock {
-    override fun read() = PlatformSystemClock.read()
-
-    override fun equals(other: Any?): Boolean {
-        return other is SystemClock && zone == other.zone
-    }
-
-    override fun hashCode(): Int = zone.hashCode() + 1
-    override fun toString(): String = "SystemClock[$zone]"
-
-    companion object {
-        /**
-         * A system clock in the UTC time zone.
-         */
-        val UTC = SystemClock(TimeZone.UTC)
-
-        /**
-         * Get the current system time zone.
-         */
-        @Deprecated(
-            "Use TimeZone.systemDefault() instead.",
-            ReplaceWith("TimeZone.systemDefault()"),
-            DeprecationLevel.WARNING
-        )
-        fun currentZone(): TimeZone = TimeZone.systemDefault()
-    }
-}
-
-/**
- * Platform system clock implementation
- */
-internal expect object PlatformSystemClock {
-    fun read(): LongMilliseconds
-}
-
-/**
- * A clock with fixed time, suitable for testing.
- */
-class FixedClock(
-    private var millisecondsSinceUnixEpoch: LongMilliseconds = 0L.milliseconds,
-    override val zone: TimeZone = TimeZone.UTC
-) : Clock {
-    
-    operator fun plusAssign(days: LongDays) {
-        millisecondsSinceUnixEpoch += days
-    }
-
-    operator fun plusAssign(days: IntDays) {
-        millisecondsSinceUnixEpoch += days
-    }
-
-    operator fun plusAssign(hours: LongHours) {
-        millisecondsSinceUnixEpoch += hours
-    }
-
-    operator fun plusAssign(hours: IntHours) {
-        millisecondsSinceUnixEpoch += hours
-    }
-
-    operator fun plusAssign(minutes: LongMinutes) {
-        millisecondsSinceUnixEpoch += minutes
-    }
-
-    operator fun plusAssign(minutes: IntMinutes) {
-        millisecondsSinceUnixEpoch += minutes
-    }
-
-    operator fun plusAssign(seconds: LongSeconds) {
-        millisecondsSinceUnixEpoch += seconds
-    }
+    fun readInstant(): Instant
 
-    operator fun plusAssign(seconds: IntSeconds) {
-        millisecondsSinceUnixEpoch += seconds
-    }
-
-    operator fun plusAssign(milliseconds: LongMilliseconds) {
-        millisecondsSinceUnixEpoch += milliseconds
-    }
-
-    operator fun plusAssign(milliseconds: IntMilliseconds) {
-        millisecondsSinceUnixEpoch += milliseconds
-    }
-
-    operator fun minusAssign(days: LongDays) {
-        millisecondsSinceUnixEpoch -= days
-    }
-
-    operator fun minusAssign(days: IntDays) {
-        millisecondsSinceUnixEpoch -= days
-    }
-
-    operator fun minusAssign(hours: LongHours) {
-        millisecondsSinceUnixEpoch -= hours
-    }
-
-    operator fun minusAssign(hours: IntHours) {
-        millisecondsSinceUnixEpoch -= hours
-    }
-
-    operator fun minusAssign(minutes: LongMinutes) {
-        millisecondsSinceUnixEpoch -= minutes
-    }
-
-    operator fun minusAssign(minutes: IntMinutes) {
-        millisecondsSinceUnixEpoch -= minutes
-    }
-
-    operator fun minusAssign(seconds: LongSeconds) {
-        millisecondsSinceUnixEpoch -= seconds
-    }
-
-    operator fun minusAssign(seconds: IntSeconds) {
-        millisecondsSinceUnixEpoch -= seconds
-    }
-
-    operator fun minusAssign(milliseconds: LongMilliseconds) {
-        millisecondsSinceUnixEpoch -= milliseconds
-    }
-
-    operator fun minusAssign(milliseconds: IntMilliseconds) {
-        millisecondsSinceUnixEpoch -= milliseconds
-    }
-
-    override fun read(): LongMilliseconds {
-        return millisecondsSinceUnixEpoch
-    }
-
-    override fun equals(other: Any?): Boolean {
-        return other is FixedClock &&
-            millisecondsSinceUnixEpoch == other.millisecondsSinceUnixEpoch &&
-            zone == other.zone
-    }
-
-    override fun hashCode(): Int {
-        return 31 * millisecondsSinceUnixEpoch.hashCode() + zone.hashCode()
-    }
-
-    override fun toString(): String = "FixedClock[${instant()}, $zone]"
+    /**
+     * Reads the current [PlatformInstant].
+     */
+    fun readPlatformInstant(): PlatformInstant
 }
 
-@Suppress("FunctionName")
-fun FixedClock(days: LongDays, zone: TimeZone = TimeZone.UTC) = FixedClock(days.inMilliseconds, zone)
-
-@Suppress("FunctionName")
-fun FixedClock(days: IntDays, zone: TimeZone = TimeZone.UTC) = FixedClock(days.inMilliseconds, zone)
-
-@Suppress("FunctionName")
-fun FixedClock(hours: LongHours, zone: TimeZone = TimeZone.UTC) = FixedClock(hours.inMilliseconds, zone)
-
-@Suppress("FunctionName")
-fun FixedClock(hours: IntHours, zone: TimeZone = TimeZone.UTC) = FixedClock(hours.inMilliseconds, zone)
-
-@Suppress("FunctionName")
-fun FixedClock(minutes: LongMinutes, zone: TimeZone = TimeZone.UTC) = FixedClock(minutes.inMilliseconds, zone)
-
-@Suppress("FunctionName")
-fun FixedClock(minutes: IntMinutes, zone: TimeZone = TimeZone.UTC) = FixedClock(minutes.inMilliseconds, zone)
-
-@Suppress("FunctionName")
-fun FixedClock(seconds: LongSeconds, zone: TimeZone = TimeZone.UTC) = FixedClock(seconds.inMilliseconds, zone)
-
-@Suppress("FunctionName")
-fun FixedClock(seconds: IntSeconds, zone: TimeZone = TimeZone.UTC) = FixedClock(seconds.inMilliseconds, zone)
-
-@Suppress("FunctionName")
-fun FixedClock(milliseconds: IntMilliseconds, zone: TimeZone = TimeZone.UTC) =
-    FixedClock(milliseconds.toLongMilliseconds(), zone)
\ No newline at end of file
+@Deprecated(
+    message = "Specify the instant explicitly.",
+    replaceWith = ReplaceWith(
+        "FixedClock(Instant(days.inSeconds), zone)",
+        "io.islandtime.Instant"
+    ),
+    level = DeprecationLevel.ERROR
+)
+fun FixedClock(days: LongDays, zone: TimeZone = TimeZone.UTC): FixedClock = deprecatedToError()
+
+@Deprecated(
+    message = "Specify the instant explicitly.",
+    replaceWith = ReplaceWith(
+        "FixedClock(Instant(days.toLong().inSeconds), zone)",
+        "io.islandtime.Instant"
+    ),
+    level = DeprecationLevel.ERROR
+)
+fun FixedClock(days: IntDays, zone: TimeZone = TimeZone.UTC): FixedClock = deprecatedToError()
+
+@Deprecated(
+    message = "Specify the instant explicitly.",
+    replaceWith = ReplaceWith(
+        "FixedClock(Instant(hours.inSeconds), zone)",
+        "io.islandtime.Instant"
+    ),
+    level = DeprecationLevel.ERROR
+)
+fun FixedClock(hours: LongHours, zone: TimeZone = TimeZone.UTC): FixedClock = deprecatedToError()
+
+@Deprecated(
+    message = "Specify the instant explicitly.",
+    replaceWith = ReplaceWith(
+        "FixedClock(Instant(hours.toLong().inSeconds), zone)",
+        "io.islandtime.Instant"
+    ),
+    level = DeprecationLevel.ERROR
+)
+fun FixedClock(hours: IntHours, zone: TimeZone = TimeZone.UTC): FixedClock = deprecatedToError()
+
+@Deprecated(
+    message = "Specify the instant explicitly.",
+    replaceWith = ReplaceWith(
+        "FixedClock(Instant(minutes.inSeconds), zone)",
+        "io.islandtime.Instant"
+    ),
+    level = DeprecationLevel.ERROR
+)
+fun FixedClock(minutes: LongMinutes, zone: TimeZone = TimeZone.UTC): FixedClock = deprecatedToError()
+
+@Deprecated(
+    message = "Specify the instant explicitly.",
+    replaceWith = ReplaceWith(
+        "FixedClock(Instant(minutes.toLong().inSeconds), zone)",
+        "io.islandtime.Instant"
+    ),
+    level = DeprecationLevel.ERROR
+)
+fun FixedClock(minutes: IntMinutes, zone: TimeZone = TimeZone.UTC): FixedClock = deprecatedToError()
+
+@Deprecated(
+    message = "Specify the instant explicitly.",
+    replaceWith = ReplaceWith(
+        "FixedClock(Instant(seconds), zone)",
+        "io.islandtime.Instant"
+    ),
+    level = DeprecationLevel.ERROR
+)
+fun FixedClock(seconds: LongSeconds, zone: TimeZone = TimeZone.UTC): FixedClock = deprecatedToError()
+
+@Deprecated(
+    message = "Specify the instant explicitly.",
+    replaceWith = ReplaceWith(
+        "FixedClock(Instant(seconds.toLongSeconds()), zone)",
+        "io.islandtime.Instant"
+    ),
+    level = DeprecationLevel.ERROR
+)
+fun FixedClock(seconds: IntSeconds, zone: TimeZone = TimeZone.UTC): FixedClock = deprecatedToError()
+
+@Deprecated(
+    message = "Specify the instant explicitly.",
+    replaceWith = ReplaceWith(
+        "FixedClock(Instant(milliseconds.toLongMilliseconds()), zone)",
+        "io.islandtime.Instant"
+    ),
+    level = DeprecationLevel.ERROR
+)
+fun FixedClock(milliseconds: IntMilliseconds, zone: TimeZone = TimeZone.UTC): FixedClock = deprecatedToError()
\ No newline at end of file
diff --git a/core/src/commonMain/kotlin/io/islandtime/clock/FixedClock.kt b/core/src/commonMain/kotlin/io/islandtime/clock/FixedClock.kt
new file mode 100644
index 000000000..2e946b565
--- /dev/null
+++ b/core/src/commonMain/kotlin/io/islandtime/clock/FixedClock.kt
@@ -0,0 +1,159 @@
+package io.islandtime.clock
+
+import io.islandtime.Instant
+import io.islandtime.PlatformInstant
+import io.islandtime.TimeZone
+import io.islandtime.internal.toPlatformInstant
+import io.islandtime.measures.*
+
+/**
+ * A clock with a fixed time, suitable for testing.
+ *
+ * @param instant the initial instant that the clock should be set to
+ * @param zone the time zone
+ */
+class FixedClock(
+    private var instant: Instant,
+    override val zone: TimeZone
+) : Clock {
+
+    fun setTo(instant: Instant) {
+        this.instant = instant
+    }
+
+    operator fun plusAssign(days: LongDays) {
+        instant += days
+    }
+
+    operator fun plusAssign(days: IntDays) {
+        instant += days
+    }
+
+    operator fun plusAssign(hours: LongHours) {
+        instant += hours
+    }
+
+    operator fun plusAssign(hours: IntHours) {
+        instant += hours
+    }
+
+    operator fun plusAssign(minutes: LongMinutes) {
+        instant += minutes
+    }
+
+    operator fun plusAssign(minutes: IntMinutes) {
+        instant += minutes
+    }
+
+    operator fun plusAssign(seconds: LongSeconds) {
+        instant += seconds
+    }
+
+    operator fun plusAssign(seconds: IntSeconds) {
+        instant += seconds
+    }
+
+    operator fun plusAssign(milliseconds: LongMilliseconds) {
+        instant += milliseconds
+    }
+
+    operator fun plusAssign(milliseconds: IntMilliseconds) {
+        instant += milliseconds
+    }
+
+    operator fun plusAssign(microseconds: LongMicroseconds) {
+        instant += microseconds
+    }
+
+    operator fun plusAssign(microseconds: IntMicroseconds) {
+        instant += microseconds
+    }
+
+    operator fun plusAssign(nanoseconds: LongNanoseconds) {
+        instant += nanoseconds
+    }
+
+    operator fun plusAssign(nanoseconds: IntNanoseconds) {
+        instant += nanoseconds
+    }
+
+    operator fun minusAssign(days: LongDays) {
+        instant -= days
+    }
+
+    operator fun minusAssign(days: IntDays) {
+        instant -= days
+    }
+
+    operator fun minusAssign(hours: LongHours) {
+        instant -= hours
+    }
+
+    operator fun minusAssign(hours: IntHours) {
+        instant -= hours
+    }
+
+    operator fun minusAssign(minutes: LongMinutes) {
+        instant -= minutes
+    }
+
+    operator fun minusAssign(minutes: IntMinutes) {
+        instant -= minutes
+    }
+
+    operator fun minusAssign(seconds: LongSeconds) {
+        instant -= seconds
+    }
+
+    operator fun minusAssign(seconds: IntSeconds) {
+        instant -= seconds
+    }
+
+    operator fun minusAssign(milliseconds: LongMilliseconds) {
+        instant -= milliseconds
+    }
+
+    operator fun minusAssign(milliseconds: IntMilliseconds) {
+        instant -= milliseconds
+    }
+
+    operator fun minusAssign(microseconds: LongMicroseconds) {
+        instant -= microseconds
+    }
+
+    operator fun minusAssign(microseconds: IntMicroseconds) {
+        instant -= microseconds
+    }
+
+    operator fun minusAssign(nanoseconds: LongNanoseconds) {
+        instant -= nanoseconds
+    }
+
+    operator fun minusAssign(nanoseconds: IntNanoseconds) {
+        instant -= nanoseconds
+    }
+
+    override fun readMilliseconds(): LongMilliseconds {
+        return instant.millisecondsSinceUnixEpoch
+    }
+
+    override fun readInstant(): Instant {
+        return instant
+    }
+
+    override fun readPlatformInstant(): PlatformInstant {
+        return instant.toPlatformInstant()
+    }
+
+    override fun equals(other: Any?): Boolean {
+        return other is FixedClock &&
+            instant == other.instant &&
+            zone == other.zone
+    }
+
+    override fun hashCode(): Int {
+        return 31 * instant.hashCode() + zone.hashCode()
+    }
+
+    override fun toString(): String = "FixedClock[$instant, $zone]"
+}
\ No newline at end of file
diff --git a/core/src/commonMain/kotlin/io/islandtime/clock/Now.kt b/core/src/commonMain/kotlin/io/islandtime/clock/Now.kt
index c11a11881..6d2f5a946 100644
--- a/core/src/commonMain/kotlin/io/islandtime/clock/Now.kt
+++ b/core/src/commonMain/kotlin/io/islandtime/clock/Now.kt
@@ -1,139 +1,103 @@
 package io.islandtime.clock
 
 import io.islandtime.*
-import io.islandtime.internal.*
-import io.islandtime.internal.MILLISECONDS_PER_DAY
-import io.islandtime.internal.MILLISECONDS_PER_SECOND
-import io.islandtime.internal.SECONDS_PER_DAY
-import io.islandtime.internal.floorDiv
-import io.islandtime.measures.nanoseconds
+import io.islandtime.clock.internal.nowImpl
+import io.islandtime.internal.deprecatedToError
 
 /**
- * Get the current [Instant] from the system clock.
+ * Gets the current [Instant] from the system clock.
  */
-fun Instant.Companion.now() = now(SystemClock.UTC)
+fun Instant.Companion.now(): Instant = now(SystemClock.UTC)
 
 /**
- * Get the current [Instant] from the specified clock.
+ * Gets the current [Instant] from the provided [clock].
  */
-fun Instant.Companion.now(clock: Clock) = clock.instant()
+fun Instant.Companion.now(clock: Clock): Instant = clock.readInstant()
 
 /**
- * Get the current [Year] from the system clock.
+ * Gets the current [Year] from the system clock.
  */
-fun Year.Companion.now() = now(SystemClock())
+fun Year.Companion.now(): Year = now(SystemClock())
 
 /**
- * Get the current [Year] from the specified clock.
+ * Gets the current [Year] from the provided [clock].
  */
-fun Year.Companion.now(clock: Clock) = Date.now(clock).toYear()
+fun Year.Companion.now(clock: Clock): Year = Date.now(clock).toYear()
 
 /**
- * Get the current [YearMonth] from the system clock.
+ * Gets the current [YearMonth] from the system clock.
  */
-fun YearMonth.Companion.now() = now(SystemClock())
+fun YearMonth.Companion.now(): YearMonth = now(SystemClock())
 
 /**
- * Get the current [YearMonth] from the specified clock.
+ * Gets the current [YearMonth] from the provided [clock].
  */
-fun YearMonth.Companion.now(clock: Clock) = Date.now(clock).toYearMonth()
+fun YearMonth.Companion.now(clock: Clock): YearMonth = Date.now(clock).toYearMonth()
 
 /**
- * Get the current [Date] from the system clock.
+ * Gets the current [Date] from the system clock.
  */
-fun Date.Companion.now() = now(SystemClock())
+fun Date.Companion.now(): Date = nowImpl(SystemClock())
 
 /**
- * Get the current [Date] from the specified clock.
+ * Gets the current [Date] from the provided [clock].
  */
-fun Date.Companion.now(clock: Clock): Date {
-    val milliseconds = clock.read()
-    val offset = clock.zone.rules.offsetAt(milliseconds)
-    val unixEpochSecond = (milliseconds.value floorDiv MILLISECONDS_PER_SECOND) + offset.totalSeconds.value
-    val unixEpochDay = unixEpochSecond floorDiv SECONDS_PER_DAY
-    return fromDayOfUnixEpoch(unixEpochDay)
-}
+fun Date.Companion.now(clock: Clock): Date = nowImpl(clock)
 
 /**
- * Get the current [DateTime] from the system clock.
+ * Gets the current [DateTime] from the system clock.
  */
-fun DateTime.Companion.now() = now(SystemClock())
+fun DateTime.Companion.now(): DateTime = nowImpl(SystemClock())
 
 /**
- * Get the current [DateTime] from the specified clock.
+ * Gets the current [DateTime] from the provided [clock].
  */
-fun DateTime.Companion.now(clock: Clock): DateTime {
-    val milliseconds = clock.read()
-    val offset = clock.zone.rules.offsetAt(milliseconds)
-    return fromMillisecondsSinceUnixEpoch(milliseconds, offset)
-}
+fun DateTime.Companion.now(clock: Clock): DateTime = nowImpl(clock)
 
 /**
- * Get the current [OffsetDateTime] from the system clock.
+ * Gets the current [OffsetDateTime] from the system clock.
  */
-fun OffsetDateTime.Companion.now() = now(SystemClock())
+fun OffsetDateTime.Companion.now(): OffsetDateTime = nowImpl(SystemClock())
 
 /**
- * Get the current [OffsetDateTime] from the specified clock.
+ * Gets the current [OffsetDateTime] from the provided [clock].
  */
-fun OffsetDateTime.Companion.now(clock: Clock): OffsetDateTime {
-    val milliseconds = clock.read()
-    val offset = clock.zone.rules.offsetAt(milliseconds)
-    return DateTime.fromMillisecondsSinceUnixEpoch(milliseconds, offset) at offset
-}
+fun OffsetDateTime.Companion.now(clock: Clock): OffsetDateTime = nowImpl(clock)
 
 /**
- * Get the current [OffsetTime] from the system clock.
+ * Gets the current [ZonedDateTime] from the system clock.
  */
-fun OffsetTime.Companion.now() = now(SystemClock())
+fun ZonedDateTime.Companion.now(): ZonedDateTime = nowImpl(SystemClock())
 
 /**
- * Get the current [OffsetTime] from the specified clock.
+ * Gets the current [ZonedDateTime] from the provided [clock].
  */
-fun OffsetTime.Companion.now(clock: Clock): OffsetTime {
-    val milliseconds = clock.read()
-    val offset = clock.zone.rules.offsetAt(milliseconds)
-
-    val nanosecondsSinceStartOfDay = ((milliseconds % MILLISECONDS_PER_DAY).inNanoseconds +
-        offset.totalSeconds + NANOSECONDS_PER_DAY.nanoseconds) % NANOSECONDS_PER_DAY
-
-    return Time.fromNanosecondOfDay(nanosecondsSinceStartOfDay.value) at offset
-}
+fun ZonedDateTime.Companion.now(clock: Clock): ZonedDateTime = nowImpl(clock)
 
 /**
- * Get the current [Time] from the system clock.
+ * Gets the current [Time] from the system clock.
  */
-fun Time.Companion.now() = now(SystemClock())
+fun Time.Companion.now(): Time = nowImpl(SystemClock())
 
 /**
- * Get the current [Time] from the specified clock.
+ * Gets the current [Time] from the provided [clock].
  */
-fun Time.Companion.now(clock: Clock): Time {
-    val milliseconds = clock.read()
-    val offset = clock.zone.rules.offsetAt(milliseconds)
-
-    val nanosecondsSinceStartOfDay = ((milliseconds % MILLISECONDS_PER_DAY).inNanoseconds +
-        offset.totalSeconds + NANOSECONDS_PER_DAY.nanoseconds) % NANOSECONDS_PER_DAY
-
-    return fromNanosecondOfDay(nanosecondsSinceStartOfDay.value)
-}
+fun Time.Companion.now(clock: Clock): Time = nowImpl(clock)
 
 /**
- * Get the current [ZonedDateTime] from the system clock.
+ * Gets the current [OffsetTime] from the system clock.
  */
-fun ZonedDateTime.Companion.now() = now(SystemClock())
+fun OffsetTime.Companion.now(): OffsetTime = nowImpl(SystemClock())
 
 /**
- * Get the current [ZonedDateTime] from the specified clock.
+ * Gets the current [OffsetTime] from the provided [clock].
  */
-fun ZonedDateTime.Companion.now(clock: Clock): ZonedDateTime {
-    return fromMillisecondsSinceUnixEpoch(clock.read(), clock.zone)
-}
+fun OffsetTime.Companion.now(clock: Clock): OffsetTime = nowImpl(clock)
 
 @Suppress("EXTENSION_SHADOWED_BY_MEMBER")
 @Deprecated(
     "Moved to TimeZone companion object.",
     ReplaceWith("systemDefault()"),
-    DeprecationLevel.WARNING
+    DeprecationLevel.ERROR
 )
-fun TimeZone.Companion.systemDefault() = systemDefault()
\ No newline at end of file
+fun TimeZone.Companion.systemDefault(): TimeZone = deprecatedToError()
\ No newline at end of file
diff --git a/core/src/commonMain/kotlin/io/islandtime/clock/SystemClock.kt b/core/src/commonMain/kotlin/io/islandtime/clock/SystemClock.kt
new file mode 100644
index 000000000..142474543
--- /dev/null
+++ b/core/src/commonMain/kotlin/io/islandtime/clock/SystemClock.kt
@@ -0,0 +1,42 @@
+@file:Suppress("FunctionName")
+
+package io.islandtime.clock
+
+import io.islandtime.TimeZone
+import io.islandtime.clock.internal.createSystemClock
+import io.islandtime.internal.deprecatedToError
+
+/**
+ * A clock that provides the time from the current system.
+ *
+ * The time zone is treated as an immutable property of the clock, set when it is created. If you wish to follow
+ * changes to the system clock's configured time zone, you must create a new [SystemClock] in response to any time zone
+ * changes.
+ */
+abstract class SystemClock protected constructor() : Clock {
+    override fun equals(other: Any?): Boolean {
+        return other is SystemClock && zone == other.zone
+    }
+
+    override fun hashCode(): Int = zone.hashCode() + 1
+    override fun toString(): String = "SystemClock[$zone]"
+
+    companion object {
+        /**
+         * A system clock in the UTC time zone.
+         */
+        val UTC: SystemClock = createSystemClock(TimeZone.UTC)
+
+        @Deprecated(
+            "Use TimeZone.systemDefault() instead.",
+            ReplaceWith("TimeZone.systemDefault()"),
+            DeprecationLevel.ERROR
+        )
+        fun currentZone(): TimeZone = deprecatedToError()
+    }
+}
+
+/**
+ * Creates a [SystemClock], optionally overriding the system's default time zone with another [zone].
+ */
+fun SystemClock(zone: TimeZone = TimeZone.systemDefault()): SystemClock = createSystemClock(zone)
\ No newline at end of file
diff --git a/core/src/commonMain/kotlin/io/islandtime/clock/internal/NowImpl.kt b/core/src/commonMain/kotlin/io/islandtime/clock/internal/NowImpl.kt
new file mode 100644
index 000000000..7c25cf93c
--- /dev/null
+++ b/core/src/commonMain/kotlin/io/islandtime/clock/internal/NowImpl.kt
@@ -0,0 +1,11 @@
+package io.islandtime.clock.internal
+
+import io.islandtime.*
+import io.islandtime.clock.Clock
+
+expect fun Date.Companion.nowImpl(clock: Clock): Date
+expect fun DateTime.Companion.nowImpl(clock: Clock): DateTime
+expect fun OffsetDateTime.Companion.nowImpl(clock: Clock): OffsetDateTime
+expect fun ZonedDateTime.Companion.nowImpl(clock: Clock): ZonedDateTime
+expect fun Time.Companion.nowImpl(clock: Clock): Time
+expect fun OffsetTime.Companion.nowImpl(clock: Clock): OffsetTime
\ No newline at end of file
diff --git a/core/src/commonMain/kotlin/io/islandtime/clock/internal/SystemClockImpl.kt b/core/src/commonMain/kotlin/io/islandtime/clock/internal/SystemClockImpl.kt
new file mode 100644
index 000000000..80ec08363
--- /dev/null
+++ b/core/src/commonMain/kotlin/io/islandtime/clock/internal/SystemClockImpl.kt
@@ -0,0 +1,6 @@
+package io.islandtime.clock.internal
+
+import io.islandtime.TimeZone
+import io.islandtime.clock.SystemClock
+
+internal expect fun createSystemClock(zone: TimeZone): SystemClock
\ No newline at end of file
diff --git a/core/src/commonMain/kotlin/io/islandtime/internal/Deprecation.kt b/core/src/commonMain/kotlin/io/islandtime/internal/Deprecation.kt
new file mode 100644
index 000000000..73c18298b
--- /dev/null
+++ b/core/src/commonMain/kotlin/io/islandtime/internal/Deprecation.kt
@@ -0,0 +1,3 @@
+package io.islandtime.internal
+
+internal fun deprecatedToError(): Nothing = throw NotImplementedError("Deprecation level is ERROR")
\ No newline at end of file
diff --git a/core/src/commonMain/kotlin/io/islandtime/internal/Instants.kt b/core/src/commonMain/kotlin/io/islandtime/internal/Instants.kt
new file mode 100644
index 000000000..aa34e0016
--- /dev/null
+++ b/core/src/commonMain/kotlin/io/islandtime/internal/Instants.kt
@@ -0,0 +1,26 @@
+package io.islandtime.internal
+
+import io.islandtime.*
+
+internal fun toTimeAt(offset: UtcOffset, secondOfUnixEpoch: Long, nanosecond: Int): Time {
+    val nanosecondOfDay = ((secondOfUnixEpoch % SECONDS_PER_DAY) * NANOSECONDS_PER_SECOND
+        + nanosecond + offset.totalSeconds.inNanoseconds.value + NANOSECONDS_PER_DAY) % NANOSECONDS_PER_DAY
+    return Time.fromNanosecondOfDay(nanosecondOfDay)
+}
+
+internal fun toOffsetTimeAt(offset: UtcOffset, secondOfUnixEpoch: Long, nanosecond: Int): OffsetTime {
+    return toTimeAt(offset, secondOfUnixEpoch, nanosecond) at offset
+}
+
+internal fun toDateAt(offset: UtcOffset, secondOfUnixEpoch: Long): Date {
+    val dayOfUnixEpoch = (secondOfUnixEpoch + offset.totalSeconds.value) floorDiv SECONDS_PER_DAY
+    return Date.fromDayOfUnixEpoch(dayOfUnixEpoch)
+}
+
+internal fun toDateTimeAt(offset: UtcOffset, secondOfUnixEpoch: Long, nanosecond: Int): DateTime {
+    return DateTime.fromSecondOfUnixEpoch(secondOfUnixEpoch, nanosecond, offset)
+}
+
+internal fun toOffsetDateTimeAt(offset: UtcOffset, secondOfUnixEpoch: Long, nanosecond: Int): OffsetDateTime {
+    return OffsetDateTime.fromSecondOfUnixEpoch(secondOfUnixEpoch, nanosecond, offset)
+}
\ No newline at end of file
diff --git a/core/src/commonMain/kotlin/io/islandtime/internal/Platform.kt b/core/src/commonMain/kotlin/io/islandtime/internal/Platform.kt
deleted file mode 100644
index 48c65c7b0..000000000
--- a/core/src/commonMain/kotlin/io/islandtime/internal/Platform.kt
+++ /dev/null
@@ -1,5 +0,0 @@
-package io.islandtime.internal
-
-import io.islandtime.TimeZone
-
-internal expect fun systemDefaultTimeZone(): TimeZone
\ No newline at end of file
diff --git a/core/src/commonMain/kotlin/io/islandtime/internal/PlatformImpl.kt b/core/src/commonMain/kotlin/io/islandtime/internal/PlatformImpl.kt
new file mode 100644
index 000000000..7b62d15fc
--- /dev/null
+++ b/core/src/commonMain/kotlin/io/islandtime/internal/PlatformImpl.kt
@@ -0,0 +1,10 @@
+package io.islandtime.internal
+
+import io.islandtime.Instant
+import io.islandtime.PlatformInstant
+import io.islandtime.TimeZone
+
+internal expect fun systemDefaultTimeZone(): TimeZone
+
+internal expect fun PlatformInstant.toIslandInstant(): Instant
+internal expect fun Instant.toPlatformInstant(): PlatformInstant
\ No newline at end of file
diff --git a/core/src/commonMain/kotlin/io/islandtime/zone/TimeZoneRules.kt b/core/src/commonMain/kotlin/io/islandtime/zone/TimeZoneRules.kt
index 1c2e5ceb2..8acec027b 100644
--- a/core/src/commonMain/kotlin/io/islandtime/zone/TimeZoneRules.kt
+++ b/core/src/commonMain/kotlin/io/islandtime/zone/TimeZoneRules.kt
@@ -64,12 +64,12 @@ interface TimeZoneOffsetTransition {
     val dateTimeAfter: DateTime
 
     /**
-     * Check if this is a gap, meaning that there are clock times that go "missing".
+     * Checks if this is a gap, meaning that there are clock times that go "missing".
      */
     val isGap: Boolean
 
     /**
-     * Check if this is an overlap, meaning that there are clock times that exist twice.
+     * Checks if this is an overlap, meaning that there are clock times that exist twice.
      */
     val isOverlap: Boolean
 
@@ -89,7 +89,7 @@ interface TimeZoneOffsetTransition {
     val duration: IntSeconds
 
     /**
-     * Get a list of the valid offsets during this transition. If this is gap, the list will be empty. If this is an
+     * Gets a list of the valid offsets during this transition. If this is gap, the list will be empty. If this is an
      * overlap, the list will contain both the earlier and later offsets.
      */
     val validOffsets: List<UtcOffset>
@@ -101,52 +101,57 @@ interface TimeZoneOffsetTransition {
  */
 interface TimeZoneRules {
     /**
-     * Check if the time zone has a fixed offset from UTC.
+     * Checks if the time zone has a fixed offset from UTC.
      */
     val hasFixedOffset: Boolean
 
     /**
-     * Get the offset in effect at a certain number of milliseconds since the Unix epoch.
+     * Gets the offset in effect at a certain number of milliseconds since the Unix epoch.
      */
     fun offsetAt(millisecondsSinceUnixEpoch: LongMilliseconds): UtcOffset
 
     /**
-     * Get the offset in effect at a certain number of seconds since the Unix epoch.
+     * Gets the offset in effect at a certain number of seconds since the Unix epoch.
      */
     fun offsetAt(secondsSinceUnixEpoch: LongSeconds, nanoOfSeconds: IntNanoseconds): UtcOffset
 
     /**
-     * Get the offset in effect at a particular instant.
+     * Gets the offset in effect at a particular instant.
      */
     fun offsetAt(instant: Instant): UtcOffset
 
     /**
-     * Get the offset in effect at a particular date and time.
+     * Gets the offset in effect at a particular instant.
+     */
+    fun offsetAt(instant: PlatformInstant): UtcOffset
+
+    /**
+     * Gets the offset in effect at a particular date and time.
      */
     fun offsetAt(dateTime: DateTime): UtcOffset
 
     /**
-     * Get a list of the valid offsets at a particular date and time.
+     * Gets a list of the valid offsets at a particular date and time.
      */
     fun validOffsetsAt(dateTime: DateTime): List<UtcOffset>
 
     /**
-     * Get the transition at a particular date and time, if one exists.
+     * Gets the transition at a particular date and time, if one exists.
      */
     fun transitionAt(dateTime: DateTime): TimeZoneOffsetTransition?
 
     /**
-     * Check if [offset] is valid at particular date and time.
+     * Checks if [offset] is valid at particular date and time.
      */
     fun isValidOffset(dateTime: DateTime, offset: UtcOffset): Boolean = validOffsetsAt(dateTime).contains(offset)
 
     /**
-     * Check if daylight savings time is in effect at a particular instant.
+     * Checks if daylight savings time is in effect at a particular instant.
      */
     fun isDaylightSavingsAt(instant: Instant): Boolean
 
     /**
-     * Get the amount of daylight savings time in effect at a particular instant. This is the amount of time added to
+     * Gets the amount of daylight savings time in effect at a particular instant. This is the amount of time added to
      * the standard offset.
      */
     fun daylightSavingsAt(instant: Instant): IntSeconds
@@ -165,6 +170,7 @@ internal class FixedTimeZoneRules(private val offset: UtcOffset) : TimeZoneRules
     }
 
     override fun offsetAt(instant: Instant) = offset
+    override fun offsetAt(instant: PlatformInstant) = offset
     override fun offsetAt(dateTime: DateTime) = offset
     override fun validOffsetsAt(dateTime: DateTime) = listOf(offset)
     override fun transitionAt(dateTime: DateTime): TimeZoneOffsetTransition? = null
diff --git a/core/src/commonTest/kotlin/io/islandtime/clock/ClockTest.kt b/core/src/commonTest/kotlin/io/islandtime/clock/ClockTest.kt
index bf76dba8f..8a478cbe9 100644
--- a/core/src/commonTest/kotlin/io/islandtime/clock/ClockTest.kt
+++ b/core/src/commonTest/kotlin/io/islandtime/clock/ClockTest.kt
@@ -13,8 +13,8 @@ class ClockTest {
     fun `UTC SystemClock`() {
         val clock = SystemClock.UTC
 
-        assertTrue { clock.read() > 0L.milliseconds }
-        assertTrue { clock.instant() > Instant.UNIX_EPOCH }
+        assertTrue { clock.readMilliseconds() > 0L.milliseconds }
+        assertTrue { clock.readInstant() > Instant.UNIX_EPOCH }
         assertEquals(TimeZone.UTC, clock.zone)
     }
 
@@ -22,8 +22,8 @@ class ClockTest {
     fun `SystemClock() without zone`() {
         val clock = SystemClock()
 
-        assertTrue { clock.read() > 0L.milliseconds }
-        assertTrue { clock.instant() > Instant.UNIX_EPOCH }
+        assertTrue { clock.readMilliseconds() > 0L.milliseconds }
+        assertTrue { clock.readInstant() > Instant.UNIX_EPOCH }
         assertNotEquals("", clock.zone.id)
     }
 
@@ -31,8 +31,8 @@ class ClockTest {
     fun `SystemClock() with zone`() {
         val clock = SystemClock(TimeZone("America/Denver"))
 
-        assertTrue { clock.read() > 0L.milliseconds }
-        assertTrue { clock.instant() > Instant.UNIX_EPOCH }
+        assertTrue { clock.readMilliseconds() > 0L.milliseconds }
+        assertTrue { clock.readInstant() > Instant.UNIX_EPOCH }
         assertEquals(TimeZone("America/Denver"), clock.zone)
     }
 }
\ No newline at end of file
diff --git a/core/src/commonTest/kotlin/io/islandtime/clock/NowTest.kt b/core/src/commonTest/kotlin/io/islandtime/clock/NowTest.kt
index 6d1f6dae3..907575265 100644
--- a/core/src/commonTest/kotlin/io/islandtime/clock/NowTest.kt
+++ b/core/src/commonTest/kotlin/io/islandtime/clock/NowTest.kt
@@ -11,7 +11,7 @@ import kotlin.test.assertEquals
 class NowTest : AbstractIslandTimeTest() {
     @Test
     fun `Year_now()`() {
-        val clock = FixedClock((-1L).milliseconds)
+        val clock = FixedClock(Instant((-1L).milliseconds), TimeZone.UTC)
         assertEquals(Year(1969), Year.now(clock))
 
         clock += 1.milliseconds
@@ -20,7 +20,7 @@ class NowTest : AbstractIslandTimeTest() {
 
     @Test
     fun `YearMonth_now()`() {
-        val clock = FixedClock((-1L).milliseconds)
+        val clock = FixedClock(Instant((-1L).milliseconds), TimeZone.UTC)
         assertEquals(YearMonth(1969, Month.DECEMBER), YearMonth.now(clock))
 
         clock += 1.milliseconds
@@ -29,7 +29,7 @@ class NowTest : AbstractIslandTimeTest() {
 
     @Test
     fun `Date_now() in UTC`() {
-        val clock = FixedClock((-1L).milliseconds)
+        val clock = FixedClock(Instant((-1L).milliseconds), TimeZone.UTC)
         assertEquals(Date(1969, Month.DECEMBER, 31), Date.now(clock))
 
         clock += 1.milliseconds
@@ -38,13 +38,13 @@ class NowTest : AbstractIslandTimeTest() {
 
     @Test
     fun `Date_now() with offset`() {
-        val clock = FixedClock((-1).days, TimeZone("Etc/GMT+1"))
+        val clock = FixedClock(Instant((-1L).days.inSeconds), TimeZone("Etc/GMT+1"))
         assertEquals(Date(1969, Month.DECEMBER, 30), Date.now(clock))
     }
 
     @Test
     fun `DateTime_now() in UTC`() {
-        val clock = FixedClock((-1L).milliseconds)
+        val clock = FixedClock(Instant((-1L).milliseconds), TimeZone.UTC)
         assertEquals(
             DateTime(1969, Month.DECEMBER, 31, 23, 59, 59, 999_000_000),
             DateTime.now(clock)
@@ -65,7 +65,7 @@ class NowTest : AbstractIslandTimeTest() {
 
     @Test
     fun `DateTime_now() with offset`() {
-        val clock = FixedClock((-1).days, TimeZone("Etc/GMT+1"))
+        val clock = FixedClock(Instant((-1L).days.inSeconds), TimeZone("Etc/GMT+1"))
         assertEquals(
             DateTime(1969, Month.DECEMBER, 30, 23, 0),
             DateTime.now(clock)
@@ -74,7 +74,7 @@ class NowTest : AbstractIslandTimeTest() {
 
     @Test
     fun `Time_now() in UTC`() {
-        val clock = FixedClock((-1).days)
+        val clock = FixedClock(Instant((-1L).days.inSeconds), TimeZone.UTC)
         assertEquals(Time(0, 0), Time.now(clock))
 
         clock += 1.milliseconds
@@ -92,25 +92,25 @@ class NowTest : AbstractIslandTimeTest() {
 
     @Test
     fun `Time_now() with offset`() {
-        val clock1 = FixedClock((-1).days, TimeZone("Etc/GMT+4"))
+        val clock1 = FixedClock(Instant((-1L).days.inSeconds), TimeZone("Etc/GMT+4"))
         assertEquals(Time(20, 0), Time.now(clock1))
 
-        val clock2 = FixedClock((-1).days, TimeZone("Etc/GMT-4"))
+        val clock2 = FixedClock(Instant((-1L).days.inSeconds), TimeZone("Etc/GMT-4"))
         assertEquals(Time(4, 0), Time.now(clock2))
     }
 
     @Test
     fun `OffsetTime_now()`() {
-        val clock1 = FixedClock((-1).days, TimeZone("Etc/GMT+4"))
+        val clock1 = FixedClock(Instant((-1L).days.inSeconds), TimeZone("Etc/GMT+4"))
         assertEquals(OffsetTime(Time(20, 0), (-4).hours.asUtcOffset()), OffsetTime.now(clock1))
 
-        val clock2 = FixedClock((-1).days, TimeZone("Etc/GMT-4"))
+        val clock2 = FixedClock(Instant((-1L).days.inSeconds), TimeZone("Etc/GMT-4"))
         assertEquals(OffsetTime(Time(4, 0), 4.hours.asUtcOffset()), OffsetTime.now(clock2))
     }
 
     @Test
     fun `OffsetDateTime_now()`() {
-        val clock = FixedClock((-1).days, TimeZone("Etc/GMT+1"))
+        val clock = FixedClock(Instant((-1L).days.inSeconds), TimeZone("Etc/GMT+1"))
         assertEquals(
             OffsetDateTime(
                 DateTime(1969, Month.DECEMBER, 30, 23, 0),
@@ -131,7 +131,7 @@ class NowTest : AbstractIslandTimeTest() {
 
     @Test
     fun `ZonedDateTime_now()`() {
-        val clock = FixedClock((-1).days, TimeZone("Etc/GMT+1"))
+        val clock = FixedClock(Instant((-1L).days.inSeconds), TimeZone("Etc/GMT+1"))
         assertEquals(
             ZonedDateTime(
                 DateTime(1969, Month.DECEMBER, 30, 23, 0),
diff --git a/core/src/darwinMain/kotlin/io/islandtime/PlatformInstant.kt b/core/src/darwinMain/kotlin/io/islandtime/PlatformInstant.kt
new file mode 100644
index 000000000..ba1f33177
--- /dev/null
+++ b/core/src/darwinMain/kotlin/io/islandtime/PlatformInstant.kt
@@ -0,0 +1,5 @@
+package io.islandtime
+
+import platform.Foundation.NSDate
+
+actual typealias PlatformInstant = NSDate
\ No newline at end of file
diff --git a/core/src/darwinMain/kotlin/io/islandtime/clock/PlatformSystemClock.kt b/core/src/darwinMain/kotlin/io/islandtime/clock/PlatformSystemClock.kt
deleted file mode 100644
index a6247b381..000000000
--- a/core/src/darwinMain/kotlin/io/islandtime/clock/PlatformSystemClock.kt
+++ /dev/null
@@ -1,20 +0,0 @@
-package io.islandtime.clock
-
-import io.islandtime.darwin.toIslandTimeZone
-import io.islandtime.measures.LongMilliseconds
-import io.islandtime.measures.microseconds
-import io.islandtime.measures.seconds
-import kotlinx.cinterop.*
-import platform.Foundation.*
-import platform.posix.gettimeofday
-import platform.posix.timeval
-
-internal actual object PlatformSystemClock {
-    actual fun read(): LongMilliseconds {
-        return memScoped {
-            val posixTime = alloc<timeval>()
-            gettimeofday(posixTime.ptr, null)
-            posixTime.tv_sec.seconds + posixTime.tv_usec.microseconds.inMilliseconds
-        }
-    }
-}
\ No newline at end of file
diff --git a/core/src/darwinMain/kotlin/io/islandtime/clock/internal/NowImpl.kt b/core/src/darwinMain/kotlin/io/islandtime/clock/internal/NowImpl.kt
new file mode 100644
index 000000000..b4506b769
--- /dev/null
+++ b/core/src/darwinMain/kotlin/io/islandtime/clock/internal/NowImpl.kt
@@ -0,0 +1,41 @@
+package io.islandtime.clock.internal
+
+import io.islandtime.*
+import io.islandtime.clock.Clock
+import io.islandtime.internal.*
+
+actual fun Date.Companion.nowImpl(clock: Clock): Date {
+    return with(clock) { readInstant().toDateAt(zone) }
+}
+
+actual fun DateTime.Companion.nowImpl(clock: Clock): DateTime {
+    return with(clock) { readInstant().toDateTimeAt(zone) }
+}
+
+actual fun OffsetDateTime.Companion.nowImpl(clock: Clock): OffsetDateTime {
+    return with(clock) { readInstant().toOffsetDateTimeAt(zone) }
+}
+
+actual fun ZonedDateTime.Companion.nowImpl(clock: Clock): ZonedDateTime {
+    return with(clock) { readInstant() at zone }
+}
+
+actual fun Time.Companion.nowImpl(clock: Clock): Time {
+    return with(clock) { readInstant().toTimeAt(zone) }
+}
+
+actual fun OffsetTime.Companion.nowImpl(clock: Clock): OffsetTime {
+    return with(clock) { readInstant().toOffsetTimeAt(zone) }
+}
+
+private fun Instant.toOffsetDateTimeAt(zone: TimeZone): OffsetDateTime {
+    return this at zone.rules.offsetAt(this)
+}
+
+private fun Instant.toTimeAt(offset: UtcOffset): Time = toTimeAt(offset, secondOfUnixEpoch, nanosecond)
+private fun Instant.toTimeAt(zone: TimeZone): Time = toTimeAt(zone.rules.offsetAt(this))
+
+private fun Instant.toOffsetTimeAt(offset: UtcOffset): OffsetTime =
+    toOffsetTimeAt(offset, secondOfUnixEpoch, nanosecond)
+
+private fun Instant.toOffsetTimeAt(zone: TimeZone): OffsetTime = toOffsetTimeAt(zone.rules.offsetAt(this))
\ No newline at end of file
diff --git a/core/src/darwinMain/kotlin/io/islandtime/clock/internal/SystemClockImpl.kt b/core/src/darwinMain/kotlin/io/islandtime/clock/internal/SystemClockImpl.kt
new file mode 100644
index 000000000..6a59126c9
--- /dev/null
+++ b/core/src/darwinMain/kotlin/io/islandtime/clock/internal/SystemClockImpl.kt
@@ -0,0 +1,38 @@
+package io.islandtime.clock.internal
+
+import io.islandtime.Instant
+import io.islandtime.PlatformInstant
+import io.islandtime.TimeZone
+import io.islandtime.clock.SystemClock
+import io.islandtime.measures.*
+import kotlinx.cinterop.alloc
+import kotlinx.cinterop.convert
+import kotlinx.cinterop.memScoped
+import kotlinx.cinterop.ptr
+import platform.Foundation.NSDate
+import platform.posix.gettimeofday
+import platform.posix.timeval
+
+internal actual fun createSystemClock(zone: TimeZone): SystemClock {
+    return object : SystemClock() {
+        override val zone: TimeZone = zone
+
+        override fun readMilliseconds(): LongMilliseconds {
+            return readSystemTime { seconds, microseconds -> seconds + microseconds.inMilliseconds }
+        }
+
+        override fun readInstant(): Instant {
+            return readSystemTime { seconds, microseconds -> Instant(seconds, microseconds.inNanoseconds) }
+        }
+
+        override fun readPlatformInstant(): PlatformInstant = NSDate()
+    }
+}
+
+private inline fun <T> readSystemTime(action: (seconds: LongSeconds, microseconds: IntMicroseconds) -> T): T {
+    return memScoped {
+        val posixTime = alloc<timeval>()
+        gettimeofday(posixTime.ptr, null)
+        action(posixTime.tv_sec.convert<Long>().seconds, posixTime.tv_usec.microseconds)
+    }
+}
\ No newline at end of file
diff --git a/core/src/darwinMain/kotlin/io/islandtime/internal/Platform.kt b/core/src/darwinMain/kotlin/io/islandtime/internal/Platform.kt
deleted file mode 100644
index 76b0935de..000000000
--- a/core/src/darwinMain/kotlin/io/islandtime/internal/Platform.kt
+++ /dev/null
@@ -1,8 +0,0 @@
-package io.islandtime.internal
-
-import io.islandtime.TimeZone
-import io.islandtime.darwin.toIslandTimeZone
-import platform.Foundation.NSTimeZone
-import platform.Foundation.localTimeZone
-
-internal actual fun systemDefaultTimeZone(): TimeZone = NSTimeZone.localTimeZone.toIslandTimeZone()
\ No newline at end of file
diff --git a/core/src/darwinMain/kotlin/io/islandtime/internal/PlatformImpl.kt b/core/src/darwinMain/kotlin/io/islandtime/internal/PlatformImpl.kt
new file mode 100644
index 000000000..e69df20be
--- /dev/null
+++ b/core/src/darwinMain/kotlin/io/islandtime/internal/PlatformImpl.kt
@@ -0,0 +1,15 @@
+package io.islandtime.internal
+
+import io.islandtime.Instant
+import io.islandtime.PlatformInstant
+import io.islandtime.TimeZone
+import io.islandtime.darwin.toIslandInstant
+import io.islandtime.darwin.toIslandTimeZone
+import io.islandtime.darwin.toNSDate
+import platform.Foundation.NSTimeZone
+import platform.Foundation.localTimeZone
+
+internal actual fun systemDefaultTimeZone(): TimeZone = NSTimeZone.localTimeZone.toIslandTimeZone()
+
+internal actual fun PlatformInstant.toIslandInstant(): Instant = toIslandInstant()
+internal actual fun Instant.toPlatformInstant(): PlatformInstant = toNSDate()
\ No newline at end of file
diff --git a/core/src/darwinMain/kotlin/io/islandtime/zone/TimeZoneRules.kt b/core/src/darwinMain/kotlin/io/islandtime/zone/TimeZoneRules.kt
index 26f3c111f..684036454 100644
--- a/core/src/darwinMain/kotlin/io/islandtime/zone/TimeZoneRules.kt
+++ b/core/src/darwinMain/kotlin/io/islandtime/zone/TimeZoneRules.kt
@@ -1,9 +1,6 @@
 package io.islandtime.zone
 
-import io.islandtime.DateTime
-import io.islandtime.Instant
-import io.islandtime.UtcOffset
-import io.islandtime.asUtcOffset
+import io.islandtime.*
 import io.islandtime.darwin.toIslandDateTimeAt
 import io.islandtime.darwin.toNSDate
 import io.islandtime.darwin.toNSDateComponents
@@ -69,6 +66,10 @@ private class DarwinTimeZoneRules(timeZone: NSTimeZone) : TimeZoneRules {
 
     override fun offsetAt(instant: Instant): UtcOffset = offsetAt(instant.toNSDate())
 
+    override fun offsetAt(instant: PlatformInstant): UtcOffset {
+        return timeZone.secondsFromGMTForDate(instant).convert<Int>().seconds.asUtcOffset()
+    }
+
     override fun transitionAt(dateTime: DateTime): TimeZoneOffsetTransition? {
         return transitionsInYear.use { map ->
             map.getOrPut(dateTime.year) {
@@ -98,10 +99,6 @@ private class DarwinTimeZoneRules(timeZone: NSTimeZone) : TimeZoneRules {
     override val hasFixedOffset: Boolean
         get() = timeZone.nextDaylightSavingTimeTransitionAfterDate(NSDate.distantPast) == null
 
-    private fun offsetAt(date: NSDate): UtcOffset {
-        return timeZone.secondsFromGMTForDate(date).convert<Int>().seconds.asUtcOffset()
-    }
-
     @OptIn(ExperimentalStdlibApi::class)
     private fun findTransitionsIn(year: Int): List<TimeZoneOffsetTransition> {
         val startDateComponents = NSDateComponents().also {
diff --git a/core/src/jvmMain/kotlin/io/islandtime/PlatformInstant.kt b/core/src/jvmMain/kotlin/io/islandtime/PlatformInstant.kt
new file mode 100644
index 000000000..ca16e4bb3
--- /dev/null
+++ b/core/src/jvmMain/kotlin/io/islandtime/PlatformInstant.kt
@@ -0,0 +1,3 @@
+package io.islandtime
+
+actual typealias PlatformInstant = java.time.Instant
\ No newline at end of file
diff --git a/core/src/jvmMain/kotlin/io/islandtime/clock/PlatformSystemClock.kt b/core/src/jvmMain/kotlin/io/islandtime/clock/PlatformSystemClock.kt
deleted file mode 100644
index b6d61b040..000000000
--- a/core/src/jvmMain/kotlin/io/islandtime/clock/PlatformSystemClock.kt
+++ /dev/null
@@ -1,7 +0,0 @@
-package io.islandtime.clock
-
-import io.islandtime.measures.milliseconds
-
-internal actual object PlatformSystemClock {
-    actual fun read() = System.currentTimeMillis().milliseconds
-}
\ No newline at end of file
diff --git a/core/src/jvmMain/kotlin/io/islandtime/clock/internal/Conversions.kt b/core/src/jvmMain/kotlin/io/islandtime/clock/internal/Conversions.kt
new file mode 100644
index 000000000..18059e92d
--- /dev/null
+++ b/core/src/jvmMain/kotlin/io/islandtime/clock/internal/Conversions.kt
@@ -0,0 +1,35 @@
+package io.islandtime.clock.internal
+
+import io.islandtime.*
+import io.islandtime.internal.*
+
+internal fun java.time.Instant.toTimeAt(offset: UtcOffset): Time = toTimeAt(offset, epochSecond, nano)
+internal fun java.time.Instant.toTimeAt(zone: TimeZone): Time = toTimeAt(zone.rules.offsetAt(this))
+
+internal fun java.time.Instant.toOffsetTimeAt(offset: UtcOffset): OffsetTime {
+    return toOffsetTimeAt(offset, epochSecond, nano)
+}
+
+internal fun java.time.Instant.toOffsetTimeAt(zone: TimeZone): OffsetTime {
+    return toOffsetTimeAt(zone.rules.offsetAt(this))
+}
+
+internal fun java.time.Instant.toDateAt(offset: UtcOffset): Date = toDateAt(offset, epochSecond)
+internal fun java.time.Instant.toDateAt(zone: TimeZone): Date = toDateAt(zone.rules.offsetAt(this))
+
+internal fun java.time.Instant.toDateTimeAt(offset: UtcOffset): DateTime = toDateTimeAt(offset, epochSecond, nano)
+internal fun java.time.Instant.toDateTimeAt(zone: TimeZone): DateTime = toDateTimeAt(zone.rules.offsetAt(this))
+
+internal fun java.time.Instant.toOffsetDateTimeAt(offset: UtcOffset): OffsetDateTime {
+    return toOffsetDateTimeAt(offset, epochSecond, nano)
+}
+
+internal fun java.time.Instant.toOffsetDateTimeAt(zone: TimeZone): OffsetDateTime {
+    return toOffsetDateTimeAt(zone.rules.offsetAt(this))
+}
+
+internal fun java.time.Instant.toZonedDateTimeAt(zone: TimeZone): ZonedDateTime {
+    val offset = zone.rules.offsetAt(this)
+    val dateTime = DateTime.fromSecondOfUnixEpoch(epochSecond, nano, offset)
+    return ZonedDateTime.create(dateTime, offset, zone)
+}
\ No newline at end of file
diff --git a/core/src/jvmMain/kotlin/io/islandtime/clock/internal/NowImpl.kt b/core/src/jvmMain/kotlin/io/islandtime/clock/internal/NowImpl.kt
new file mode 100644
index 000000000..881f81cd3
--- /dev/null
+++ b/core/src/jvmMain/kotlin/io/islandtime/clock/internal/NowImpl.kt
@@ -0,0 +1,28 @@
+package io.islandtime.clock.internal
+
+import io.islandtime.*
+import io.islandtime.clock.Clock
+
+actual fun Date.Companion.nowImpl(clock: Clock): Date {
+    return with(clock) { readPlatformInstant().toDateAt(zone) }
+}
+
+actual fun DateTime.Companion.nowImpl(clock: Clock): DateTime {
+    return with(clock) { readPlatformInstant().toDateTimeAt(zone) }
+}
+
+actual fun OffsetDateTime.Companion.nowImpl(clock: Clock): OffsetDateTime {
+    return with(clock) { readPlatformInstant().toOffsetDateTimeAt(zone) }
+}
+
+actual fun ZonedDateTime.Companion.nowImpl(clock: Clock): ZonedDateTime {
+    return with(clock) { readPlatformInstant().toZonedDateTimeAt(zone) }
+}
+
+actual fun Time.Companion.nowImpl(clock: Clock): Time {
+    return with(clock) { readPlatformInstant().toTimeAt(zone) }
+}
+
+actual fun OffsetTime.Companion.nowImpl(clock: Clock): OffsetTime {
+    return with(clock) { readPlatformInstant().toOffsetTimeAt(zone) }
+}
\ No newline at end of file
diff --git a/core/src/jvmMain/kotlin/io/islandtime/clock/internal/SystemClockImpl.kt b/core/src/jvmMain/kotlin/io/islandtime/clock/internal/SystemClockImpl.kt
new file mode 100644
index 000000000..8328f6c03
--- /dev/null
+++ b/core/src/jvmMain/kotlin/io/islandtime/clock/internal/SystemClockImpl.kt
@@ -0,0 +1,21 @@
+package io.islandtime.clock.internal
+
+import io.islandtime.Instant
+import io.islandtime.PlatformInstant
+import io.islandtime.TimeZone
+import io.islandtime.clock.SystemClock
+import io.islandtime.jvm.toIslandInstant
+import io.islandtime.measures.LongMilliseconds
+import io.islandtime.measures.milliseconds
+import java.time.Clock as JavaClock
+
+private val javaClock = JavaClock.systemUTC()
+
+internal actual fun createSystemClock(zone: TimeZone): SystemClock {
+    return object : SystemClock() {
+        override val zone: TimeZone = zone
+        override fun readMilliseconds(): LongMilliseconds = System.currentTimeMillis().milliseconds
+        override fun readInstant(): Instant = readPlatformInstant().toIslandInstant()
+        override fun readPlatformInstant(): PlatformInstant = javaClock.instant()
+    }
+}
\ No newline at end of file
diff --git a/core/src/jvmMain/kotlin/io/islandtime/clock/jvm/Now.kt b/core/src/jvmMain/kotlin/io/islandtime/clock/jvm/Now.kt
new file mode 100644
index 000000000..928852ed9
--- /dev/null
+++ b/core/src/jvmMain/kotlin/io/islandtime/clock/jvm/Now.kt
@@ -0,0 +1,64 @@
+package io.islandtime.clock.jvm
+
+import io.islandtime.*
+import io.islandtime.clock.internal.*
+import io.islandtime.internal.toIslandInstant
+import io.islandtime.jvm.toIslandTimeZone
+import java.time.Clock as JavaClock
+
+/**
+ * Gets the current [Instant] from the provided [clock].
+ */
+fun Instant.Companion.now(clock: JavaClock): Instant = clock.instant().toIslandInstant()
+
+/**
+ * Gets the current [Year] from the provided [clock].
+ */
+fun Year.Companion.now(clock: JavaClock): Year = Date.now(clock).toYear()
+
+/**
+ * Gets the current [YearMonth] from the provided [clock].
+ */
+fun YearMonth.Companion.now(clock: JavaClock): YearMonth = Date.now(clock).toYearMonth()
+
+/**
+ * Gets the current [Date] from the provided [clock].
+ */
+fun Date.Companion.now(clock: JavaClock): Date {
+    return with(clock) { instant().toDateAt(zone.toIslandTimeZone()) }
+}
+
+/**
+ * Gets the current [DateTime] from the provided [clock].
+ */
+fun DateTime.Companion.now(clock: JavaClock): DateTime {
+    return with(clock) { instant().toDateTimeAt(zone.toIslandTimeZone()) }
+}
+
+/**
+ * Gets the current [OffsetDateTime] from the provided [clock].
+ */
+fun OffsetDateTime.Companion.now(clock: JavaClock): OffsetDateTime {
+    return with(clock) { instant().toOffsetDateTimeAt(zone.toIslandTimeZone()) }
+}
+
+/**
+ * Gets the current [ZonedDateTime] from the provided [clock].
+ */
+fun ZonedDateTime.Companion.now(clock: JavaClock): ZonedDateTime {
+    return with(clock) { instant().toZonedDateTimeAt(zone.toIslandTimeZone()) }
+}
+
+/**
+ * Gets the current [Time] from the provided [clock].
+ */
+fun Time.Companion.now(clock: JavaClock): Time {
+    return with(clock) { instant().toTimeAt(zone.toIslandTimeZone()) }
+}
+
+/**
+ * Gets the current [OffsetTime] from the provided [clock].
+ */
+fun OffsetTime.Companion.now(clock: JavaClock): OffsetTime {
+    return with(clock) { instant().toOffsetTimeAt(zone.toIslandTimeZone()) }
+}
\ No newline at end of file
diff --git a/core/src/jvmMain/kotlin/io/islandtime/internal/Platform.kt b/core/src/jvmMain/kotlin/io/islandtime/internal/Platform.kt
deleted file mode 100644
index 0384354e3..000000000
--- a/core/src/jvmMain/kotlin/io/islandtime/internal/Platform.kt
+++ /dev/null
@@ -1,18 +0,0 @@
-package io.islandtime.internal
-
-import io.islandtime.TimeZone
-
-// TODO: Benchmark and see if using a ConcurrentHashMap would be faster than parsing the ID each time
-internal actual fun systemDefaultTimeZone(): TimeZone = java.util.TimeZone.getDefault().toIslandTimeZone()
-
-internal fun java.util.TimeZone.toIslandTimeZone(): TimeZone {
-    return if (id.startsWith("GMT")) {
-        if (id.length == 3) {
-            TimeZone.Region(id)
-        } else {
-            TimeZone.FixedOffset(id.substring(3))
-        }
-    } else {
-        TimeZone.Region(id)
-    }
-}
\ No newline at end of file
diff --git a/core/src/jvmMain/kotlin/io/islandtime/internal/PlatformImpl.kt b/core/src/jvmMain/kotlin/io/islandtime/internal/PlatformImpl.kt
new file mode 100644
index 000000000..f87d63e76
--- /dev/null
+++ b/core/src/jvmMain/kotlin/io/islandtime/internal/PlatformImpl.kt
@@ -0,0 +1,34 @@
+package io.islandtime.internal
+
+import io.islandtime.*
+import io.islandtime.jvm.toIslandInstant
+import io.islandtime.jvm.toJavaInstant
+
+internal actual fun systemDefaultTimeZone(): TimeZone = java.util.TimeZone.getDefault().toIslandTimeZone()
+
+internal actual fun PlatformInstant.toIslandInstant(): Instant = this.toIslandInstant()
+internal actual fun Instant.toPlatformInstant(): PlatformInstant = this.toJavaInstant()
+
+private var lastDefaultTimeZone: Pair<java.util.TimeZone, TimeZone>? = null
+
+internal fun java.util.TimeZone.toIslandTimeZone(): TimeZone {
+    lastDefaultTimeZone?.let { (javaZone, islandZone) ->
+        if (this == javaZone) {
+            return islandZone
+        }
+    }
+
+    val id = id
+    val islandZone = if (id.startsWith("GMT")) {
+        if (id.length == 3) {
+            TimeZone.Region(id)
+        } else {
+            TimeZone.FixedOffset(id.substring(3))
+        }
+    } else {
+        TimeZone.Region(id)
+    }
+
+    lastDefaultTimeZone = this to islandZone
+    return islandZone
+}
\ No newline at end of file
diff --git a/core/src/jvmMain/kotlin/io/islandtime/jvm/Conversions.kt b/core/src/jvmMain/kotlin/io/islandtime/jvm/Conversions.kt
index 59cf8c061..c94c71fb7 100644
--- a/core/src/jvmMain/kotlin/io/islandtime/jvm/Conversions.kt
+++ b/core/src/jvmMain/kotlin/io/islandtime/jvm/Conversions.kt
@@ -3,6 +3,7 @@
 package io.islandtime.jvm
 
 import io.islandtime.*
+import io.islandtime.clock.Clock
 import io.islandtime.internal.MICROSECONDS_PER_SECOND
 import io.islandtime.measures.*
 
@@ -349,4 +350,22 @@ fun IntNanoseconds.toJavaDuration(): java.time.Duration = java.time.Duration.ofN
 /**
  * Convert to an equivalent Java `Duration`.
  */
-fun LongNanoseconds.toJavaDuration(): java.time.Duration = java.time.Duration.ofNanos(value)
\ No newline at end of file
+fun LongNanoseconds.toJavaDuration(): java.time.Duration = java.time.Duration.ofNanos(value)
+
+/**
+ * Makes this clock compatible with Island Time's [Clock] interface.
+ */
+fun java.time.Clock.asIslandClock(): Clock = WrappedJavaClock(clock = this)
+
+private class WrappedJavaClock(private val clock: java.time.Clock) : Clock {
+    override fun equals(other: Any?): Boolean {
+        return other is WrappedJavaClock && other.clock == clock
+    }
+
+    override fun hashCode(): Int = clock.hashCode()
+    override fun toString(): String = "$clock (Java)"
+    override val zone: TimeZone get() = clock.zone.toIslandTimeZone()
+    override fun readPlatformInstant(): PlatformInstant = clock.instant()
+    override fun readInstant(): Instant = readPlatformInstant().toIslandInstant()
+    override fun readMilliseconds(): LongMilliseconds = clock.millis().milliseconds
+}
\ No newline at end of file
diff --git a/core/src/jvmMain/kotlin/io/islandtime/zone/TimeZoneRules.kt b/core/src/jvmMain/kotlin/io/islandtime/zone/TimeZoneRules.kt
index c371c5178..4e8d6be82 100644
--- a/core/src/jvmMain/kotlin/io/islandtime/zone/TimeZoneRules.kt
+++ b/core/src/jvmMain/kotlin/io/islandtime/zone/TimeZoneRules.kt
@@ -2,6 +2,7 @@ package io.islandtime.zone
 
 import io.islandtime.DateTime
 import io.islandtime.Instant
+import io.islandtime.PlatformInstant
 import io.islandtime.UtcOffset
 import io.islandtime.jvm.*
 import io.islandtime.measures.*
@@ -58,6 +59,15 @@ private class JavaTimeZoneRules(private val javaZoneRules: ZoneRules) : TimeZone
         return with(instant) { offsetAt(secondsSinceUnixEpoch, additionalNanosecondsSinceUnixEpoch) }
     }
 
+    override fun offsetAt(instant: PlatformInstant): UtcOffset {
+        return try {
+            javaZoneRules.getOffset(instant).toIslandUtcOffset()
+        } catch (e: ArithmeticException) {
+            // Workaround for Android desugaring issue (https://issuetracker.google.com/issues/153773237)
+            UtcOffset.ZERO
+        }
+    }
+
     override fun offsetAt(dateTime: DateTime): UtcOffset {
         val offset = javaZoneRules.getOffset(dateTime.toJavaLocalDateTime())
         return offset.toIslandUtcOffset()
@@ -87,15 +97,6 @@ private class JavaTimeZoneRules(private val javaZoneRules: ZoneRules) : TimeZone
     override fun daylightSavingsAt(instant: Instant): IntSeconds {
         return javaZoneRules.getDaylightSavings(instant.toJavaInstant()).seconds.toInt().seconds
     }
-
-    private fun offsetAt(javaInstant: JavaInstant): UtcOffset {
-        return try {
-            javaZoneRules.getOffset(javaInstant).toIslandUtcOffset()
-        } catch (e: ArithmeticException) {
-            // Workaround for Android desugaring issue (https://issuetracker.google.com/issues/153773237)
-            UtcOffset.ZERO
-        }
-    }
 }
 
 private class JavaTimeZoneOffsetTransition(
diff --git a/core/src/jvmTest/kotlin/io/islandtime/clock/jvm/JvmNowTest.kt b/core/src/jvmTest/kotlin/io/islandtime/clock/jvm/JvmNowTest.kt
new file mode 100644
index 000000000..a029d6072
--- /dev/null
+++ b/core/src/jvmTest/kotlin/io/islandtime/clock/jvm/JvmNowTest.kt
@@ -0,0 +1,103 @@
+package io.islandtime.clock.jvm
+
+import io.islandtime.*
+import io.islandtime.measures.hours
+import io.islandtime.test.AbstractIslandTimeTest
+import org.junit.Test
+import kotlin.test.assertEquals
+import java.time.Clock as JavaClock
+import java.time.Instant as JavaInstant
+import java.time.ZoneId as JavaZoneId
+import java.time.ZoneOffset as JavaZoneOffset
+
+class JvmNowTest : AbstractIslandTimeTest() {
+    @Test
+    fun `Year_now()`() {
+        val clock = JavaClock.fixed(JavaInstant.ofEpochMilli(-1L), JavaZoneOffset.UTC)
+        assertEquals(Year(1969), Year.now(clock))
+    }
+
+    @Test
+    fun `YearMonth_now()`() {
+        val clock = JavaClock.fixed(JavaInstant.ofEpochMilli(-1L), JavaZoneOffset.UTC)
+        assertEquals(YearMonth(1969, Month.DECEMBER), YearMonth.now(clock))
+    }
+
+    @Test
+    fun `Date_now() in UTC`() {
+        val clock = JavaClock.fixed(JavaInstant.ofEpochMilli(-1L), JavaZoneOffset.UTC)
+        assertEquals(Date(1969, Month.DECEMBER, 31), Date.now(clock))
+    }
+
+    @Test
+    fun `Date_now() with offset`() {
+        val clock = JavaClock.fixed(JavaInstant.parse("1969-12-31T00:00:00Z"), JavaZoneId.of("Etc/GMT+1"))
+        assertEquals(Date(1969, Month.DECEMBER, 30), Date.now(clock))
+    }
+
+    @Test
+    fun `DateTime_now() in UTC`() {
+        val clock = JavaClock.fixed(JavaInstant.ofEpochMilli(-1L), JavaZoneOffset.UTC)
+        assertEquals(
+            DateTime(1969, Month.DECEMBER, 31, 23, 59, 59, 999_000_000),
+            DateTime.now(clock)
+        )
+    }
+
+    @Test
+    fun `DateTime_now() with offset`() {
+        val clock = JavaClock.fixed(JavaInstant.parse("1969-12-31T00:00:00Z"), JavaZoneId.of("Etc/GMT+1"))
+        assertEquals(
+            DateTime(1969, Month.DECEMBER, 30, 23, 0),
+            DateTime.now(clock)
+        )
+    }
+
+    @Test
+    fun `Time_now() in UTC`() {
+        val clock = JavaClock.fixed(JavaInstant.parse("1969-12-31T00:00:00Z"), JavaZoneOffset.UTC)
+        assertEquals(Time(0, 0), Time.now(clock))
+    }
+
+    @Test
+    fun `Time_now() with offset`() {
+        val clock1 = JavaClock.fixed(JavaInstant.parse("1969-12-31T00:00:00Z"), JavaZoneId.of("Etc/GMT+4"))
+        assertEquals(Time(20, 0), Time.now(clock1))
+
+        val clock2 = JavaClock.fixed(JavaInstant.parse("1969-12-31T00:00:00Z"), JavaZoneId.of("Etc/GMT-4"))
+        assertEquals(Time(4, 0), Time.now(clock2))
+    }
+
+    @Test
+    fun `OffsetTime_now()`() {
+        val clock1 = JavaClock.fixed(JavaInstant.parse("1969-12-31T00:00:00Z"), JavaZoneId.of("Etc/GMT+4"))
+        assertEquals(OffsetTime(Time(20, 0), (-4).hours.asUtcOffset()), OffsetTime.now(clock1))
+
+        val clock2 = JavaClock.fixed(JavaInstant.parse("1969-12-31T00:00:00Z"), JavaZoneId.of("Etc/GMT-4"))
+        assertEquals(OffsetTime(Time(4, 0), 4.hours.asUtcOffset()), OffsetTime.now(clock2))
+    }
+
+    @Test
+    fun `OffsetDateTime_now()`() {
+        val clock = JavaClock.fixed(JavaInstant.parse("1969-12-31T00:00:00Z"), JavaZoneId.of("Etc/GMT+1"))
+        assertEquals(
+            OffsetDateTime(
+                DateTime(1969, Month.DECEMBER, 30, 23, 0),
+                (-1).hours.asUtcOffset()
+            ),
+            OffsetDateTime.now(clock)
+        )
+    }
+
+    @Test
+    fun `ZonedDateTime_now()`() {
+        val clock = JavaClock.fixed(JavaInstant.parse("1969-12-31T00:00:00Z"), JavaZoneId.of("Etc/GMT+1"))
+        assertEquals(
+            ZonedDateTime(
+                DateTime(1969, Month.DECEMBER, 30, 23, 0),
+                TimeZone("Etc/GMT+1")
+            ),
+            ZonedDateTime.now(clock)
+        )
+    }
+}
\ No newline at end of file
diff --git a/core/src/jvmTest/kotlin/io/islandtime/internal/PlatformTest.kt b/core/src/jvmTest/kotlin/io/islandtime/internal/PlatformImplTest.kt
similarity index 79%
rename from core/src/jvmTest/kotlin/io/islandtime/internal/PlatformTest.kt
rename to core/src/jvmTest/kotlin/io/islandtime/internal/PlatformImplTest.kt
index eaa4ec896..8007ead9a 100644
--- a/core/src/jvmTest/kotlin/io/islandtime/internal/PlatformTest.kt
+++ b/core/src/jvmTest/kotlin/io/islandtime/internal/PlatformImplTest.kt
@@ -8,13 +8,15 @@ import kotlin.test.assertEquals
 import kotlin.test.assertFailsWith
 import kotlin.test.assertTrue
 
-class PlatformTest : AbstractIslandTimeTest() {
+class PlatformImplTest : AbstractIslandTimeTest() {
     @Test
     fun `converts java_util_TimeZone with UTC equivalent IDs to Island region-based zones`() {
-        listOf("GMT", "UTC").forEach {
-            val tz = java.util.TimeZone.getTimeZone(it).toIslandTimeZone()
-            assertEquals(it, tz.id)
-            assertTrue { tz.isValid }
+        listOf("GMT", "UTC").forEach { id ->
+            repeat(2) {
+                val tz = java.util.TimeZone.getTimeZone(id).toIslandTimeZone()
+                assertEquals(id, tz.id)
+                assertTrue { tz.isValid }
+            }
         }
     }
 
diff --git a/core/src/jvmTest/kotlin/io/islandtime/jvm/ConversionsTest.kt b/core/src/jvmTest/kotlin/io/islandtime/jvm/ConversionsTest.kt
index a08d91482..13992aeba 100644
--- a/core/src/jvmTest/kotlin/io/islandtime/jvm/ConversionsTest.kt
+++ b/core/src/jvmTest/kotlin/io/islandtime/jvm/ConversionsTest.kt
@@ -296,4 +296,28 @@ class ConversionsTest {
         assertEquals(1_000_000_000L, 1_000_000_000.nanoseconds.toJavaDuration().toNanos())
         assertEquals(-1L, (-1L).nanoseconds.toJavaDuration().toNanos())
     }
+
+    @Test
+    fun `converts a Java Clock to an Island Time Clock`() {
+        val javaClock = java.time.Clock.fixed(
+            java.time.Instant.ofEpochSecond(234_678_901L, 123456789),
+            java.time.ZoneId.of("America/New_York")
+        )
+
+        val islandClock = javaClock.asIslandClock()
+
+        assertEquals(
+            java.time.Instant.ofEpochSecond(234_678_901L, 123456789),
+            islandClock.readPlatformInstant()
+        )
+        assertEquals(
+            Instant.fromSecondOfUnixEpoch(234_678_901L, 123456789),
+            islandClock.readInstant()
+        )
+        assertEquals(
+            Instant.fromSecondOfUnixEpoch(234_678_901L, 123456789).millisecondsSinceUnixEpoch,
+            islandClock.readMilliseconds()
+        )
+        assertEquals(TimeZone("America/New_York"), islandClock.zone)
+    }
 }
\ No newline at end of file