Add warning when using a textual date field specifier with the COMPAT locale provider

server/src/main/java/org/elasticsearch/common/time/DateFormatters.java

@@ -2323,6 +2323,8 @@ static DateFormatter forPattern(String input) {
         } else if (FormatNames.STRICT_YEAR_MONTH_DAY.matches(input)) {
             return STRICT_YEAR_MONTH_DAY;
         } else {
+            DateUtils.checkTextualDateFormats(input);
+
             try {
                 return newDateFormatter(
                     input,

server/src/main/java/org/elasticsearch/common/time/DateUtils.java

@@ -10,6 +10,9 @@
 
 import org.elasticsearch.common.logging.DeprecationCategory;
 import org.elasticsearch.common.logging.DeprecationLogger;
+import org.elasticsearch.core.Predicates;
+import org.elasticsearch.core.UpdateForV9;
+import org.elasticsearch.logging.LogManager;
 
 import java.time.Clock;
 import java.time.Duration;
@@ -19,6 +22,8 @@
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.Map;
+import java.util.function.Predicate;
+import java.util.regex.Pattern;
 
 import static java.util.Map.entry;
 import static org.elasticsearch.common.time.DateUtilsRounding.getMonthOfYear;
@@ -382,4 +387,16 @@ public static ZonedDateTime nowWithMillisResolution(Clock clock) {
         Clock millisResolutionClock = Clock.tick(clock, Duration.ofMillis(1));
         return ZonedDateTime.now(millisResolutionClock);
     }
+
+    // check for all textual fields, and localized zone offset
+    private static final Predicate<String> CONTAINS_CHANGING_TEXT_SPECIFIERS = System.getProperty("java.locale.providers", "")
+        .contains("COMPAT") ? Pattern.compile("[EcGaO]|MMM|LLL|eee|ccc|QQQ|ZZZZ").asPredicate() : Predicates.never();
+
+    @UpdateForV9    // this can be removed, we will only use CLDR on v9
+    static void checkTextualDateFormats(String format) {
+        if (CONTAINS_CHANGING_TEXT_SPECIFIERS.test(format)) {
+            LogManager.getLogger(DateFormatter.class)
+                .warn("Date format [{}] contains textual field specifiers that could change in JDK 23", format);
+        }
+    }
 }

x-pack/plugin/core/src/main/java/org/elasticsearch/license/ClusterStateLicenseService.java

@@ -140,7 +140,7 @@ CharSequence buildExpirationMessage(long expirationMillis, boolean expired) {
             License [{}] on [{}].
             # If you have a new license, please update it. Otherwise, please reach out to
             # your support contact.
-            #\s""", expiredMsg, LicenseUtils.DATE_FORMATTER.formatMillis(expirationMillis));
+            #\s""", expiredMsg, LicenseUtils.formatMillis(expirationMillis));
         if (expired) {
             general = general.toUpperCase(Locale.ROOT);
         }

x-pack/plugin/core/src/main/java/org/elasticsearch/license/LicenseUtils.java

@@ -8,14 +8,16 @@
 
 import org.elasticsearch.ElasticsearchSecurityException;
 import org.elasticsearch.common.hash.MessageDigests;
-import org.elasticsearch.common.time.DateFormatter;
 import org.elasticsearch.license.License.LicenseType;
 import org.elasticsearch.license.internal.XPackLicenseStatus;
 import org.elasticsearch.protocol.xpack.license.LicenseStatus;
 import org.elasticsearch.rest.RestStatus;
 
 import java.nio.charset.StandardCharsets;
 import java.time.Clock;
+import java.time.Instant;
+import java.time.ZoneOffset;
+import java.time.format.DateTimeFormatter;
 import java.util.HashMap;
 import java.util.Locale;
 import java.util.Map;
@@ -25,7 +27,13 @@
 public class LicenseUtils {
 
     public static final String EXPIRED_FEATURE_METADATA = "es.license.expired.feature";
-    public static final DateFormatter DATE_FORMATTER = DateFormatter.forPattern("EEEE, MMMM dd, yyyy").withLocale(Locale.ENGLISH);
+
+    public static String formatMillis(long millis) {
+        // DateFormatters logs a warning about the pattern on COMPAT
+        // this will be confusing to users, so call DateTimeFormatter directly instead
+        return DateTimeFormatter.ofPattern("EEEE, MMMM dd, yyyy", Locale.ENGLISH)
+            .format(Instant.ofEpochMilli(millis).atOffset(ZoneOffset.UTC));
+    }
 
     /**
      * Exception to be thrown when a feature action requires a valid license, but license
@@ -155,7 +163,7 @@ public static String getExpiryWarning(long licenseExpiryDate, long currentTime)
                 ? "expires today"
                 : (diff > 0
                     ? String.format(Locale.ROOT, "will expire in [%d] days", days)
-                    : String.format(Locale.ROOT, "expired on [%s]", LicenseUtils.DATE_FORMATTER.formatMillis(licenseExpiryDate)));
+                    : String.format(Locale.ROOT, "expired on [%s]", formatMillis(licenseExpiryDate)));
             return "Your license "
                 + expiryMessage
                 + ". "