Summary

java.time.Instant should be able to parse ISO 8601 offsets of the form HH:mm:ss

Problem

The spec of DateTimeFormatterBuilder.appendOffsetId() it reads:

This is equivalent to calling appendOffset("+HH:mm:ss", "Z")

and in appendOffset(String, String):

+HH:mm:ss - hour, with minute if non-zero or with minute and second if non-zero, with colon

This implies the minutes are optional, so hour-only offsets such as "+02" should be parsed correctly. However, the actual behavior is not, because the actual implementation uses "+HH:MM:ss" for parsing/formatting the zone offset.

Solution

The proposed solution consists of two parts:

• Correct the documentation of appendOffsetId(): Replace the pattern in the spec "+HH:mm:ss" with "+HH:MM:ss" to align with the implementation.
• Allow ISO_INSTANT formatter to parse ISO 8601 offsets, by explicitly specifying the offset pattern "+HH" in appendOffset(), instead of appendOffsetId(). This enables parsing both ‘+HH:mm:ss’ and ‘+HHmmss’, covering all valid ISO 8601 offset formats

Specification

Replace the pattern in DateTimeFormatterBuilder.appendOffsetId() as follows:

      * Appends the zone offset, such as '+01:00', to the formatter.
      * <p>
      * This appends an instruction to format/parse the offset ID to the builder.
-     * This is equivalent to calling {@code appendOffset("+HH:mm:ss", "Z")}.
+     * This is equivalent to calling {@code appendOffset("+HH:MM:ss", "Z")}.
      * See {@link #appendOffset(String, String)} for details on formatting
      * and parsing.
      *

Insert the following sentence in DateTimeFormatter.ISO_INSTANT:

--- a/src/java.base/share/classes/java/time/format/DateTimeFormatter.java
+++ b/src/java.base/share/classes/java/time/format/DateTimeFormatter.java
@@ -1199,8 +1199,9 @@
      * When formatting, the instant will always be suffixed by 'Z' to indicate UTC.
      * The second-of-minute is always output.
      * The nano-of-second outputs zero, three, six or nine digits as necessary.
-     * When parsing, the behaviour of {@link DateTimeFormatterBuilder#appendOffsetId()}
-     * will be used to parse the offset, converting the instant to UTC as necessary.
+     * When parsing, the lenient mode behaviour of
+     * {@link DateTimeFormatterBuilder#appendOffset(String, String)
+     * appendOffset("+HH", "Z")} in will be used to parse the offset,
+     * converting the instant to UTC as necessary.
      * The time to at least the seconds field is required.
      * Fractional seconds from zero to nine are parsed.
      * The localized decimal style is not used.

and in DateTimeFormatterBuilder.appendInstant():

--- a/src/java.base/share/classes/java/time/format/DateTimeFormatterBuilder.java
+++ b/src/java.base/share/classes/java/time/format/DateTimeFormatterBuilder.java
@@ -894,8 +894,9 @@ 
      * {@link DateTimeFormatter#parsedLeapSecond()} for full details.
      * <p>
      * When formatting, the instant will always be suffixed by 'Z' to indicate UTC.
-     * When parsing, the behaviour of {@link DateTimeFormatterBuilder#appendOffsetId()}
-     * will be used to parse the offset, converting the instant to UTC as necessary.
+     * When parsing, the lenient mode behaviour of
+     * {@link DateTimeFormatterBuilder#appendOffset(String, String)
+     * appendOffset("+HH", "Z")} in will be used to parse the offset,
+     * converting the instant to UTC as necessary.
      * <p>
      * An alternative to this method is to format/parse the instant as a single
      * epoch-seconds value. That is achieved using {@code appendValue(INSTANT_SECONDS)}.