Fix DateFormatters.parseMillis when no timezone is given

The parseMillis method was able to work on formats without timezones by
falling back to UTC. The Java Date Formatter did not support this, as
the calling code was using the `Instant.from` java time API.

This switches over to an internal method which adds UTC as a timezone.

Closes elastic#39067

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

@@ -47,7 +47,7 @@ public interface DateFormatter {
      * Parse the given input into millis-since-epoch.
      */
     default long parseMillis(String input) {
-        return Instant.from(parse(input)).toEpochMilli();
+        return DateFormatters.from(parse(input)).toInstant().toEpochMilli();
     }
 
     /**

server/src/test/java/org/elasticsearch/common/joda/JavaJodaTimeDuellingTests.java

@@ -683,6 +683,12 @@ public void testIso8601Parsers() {
         assertSameDate("2018-10-10T10:11:12,123Z", format, jodaFormatter, javaFormatter);
     }
 
+    public void testParsingMissingTimezone() {
+        long millisJava = DateFormatter.forPattern("8yyyy-MM-dd HH:mm:ss").parseMillis("2018-02-18 17:47:17");
+        long millisJoda = DateFormatter.forPattern("yyyy-MM-dd HH:mm:ss").parseMillis("2018-02-18 17:47:17");
+        assertThat(millisJava, is(millisJoda));
+    }
+
     private void assertSamePrinterOutput(String format, ZonedDateTime javaDate, DateTime jodaDate) {
         assertThat(jodaDate.getMillis(), is(javaDate.toInstant().toEpochMilli()));
         String javaTimeOut = DateFormatter.forPattern(format).format(javaDate);