ZonedDateTime toString compatability with ISO 8601

The example in the documentation is 2007-12-03T10:15:30+01:00[Europe/Paris]. This happens not to be ISO compliant since ISO-8601 does not include the [Europe/Paris] part. This was added by the java.time developers in a compromise between getting as close to the standard as reasonable and still provding the time zone information in an unambiguous way.

So the real question may in fact be the opposite: if ZonedDateTime.toString() includes the time zone information that ISO does not include, when is the result fully ISO compliant? What does “if the offset and ID are the same” mean? Here we have to remember that ZoneOffset is a subclass of ZoneID and may be used as a zone ID in ZonedDateTime. In this case the offset and the ID are the same. Otherwise they are not. For a specific example, ZonedDateTime.now(ZoneOffset.ofHours(+2)).toString() may produce 2017-04-26T15:04:59.828+02:00. This is fully ISO compatible because the zone is given as just +02:00, which is the same as the offset. Also ZonedDateTime.now(ZoneOffset.UTC).toString() gives something in the format 2017-04-26T13:04:59.828Z. Since Z counts as an offset, this is compatible too.

I think that in most cases it won’t be very useful. If your zone is just an offset, you would usually prefer to use OffsetDateTime over ZonedDateTime, and if so, of course you don’t care whether ZonedDateTime.toString() is ISO compatible or not.

This is the complete specification:

 * Outputs this date-time as a {@code String}, such as
 * {@code 2007-12-03T10:15:30+01:00[Europe/Paris]}.
 * <p>
 * The format consists of the {@code LocalDateTime} followed by the {@code ZoneOffset}.
 * If the {@code ZoneId} is not the same as the offset, then the ID is output.
 * The output is compatible with ISO-8601 if the offset and ID are the same.

The Javadoc specification refers to the case where the ZonedDateTime is constructed with a ZoneOffset rather than a named ZoneId, thus where the offset and ID are the same:

System.out.println(ZonedDateTime.now(ZoneId.of("Europe/Paris")));
// 2017-04-26T15:13:12.006+02:00[Europe/Paris]

System.out.println(ZonedDateTime.now(ZoneOffset.ofHours(2)));
// 2017-04-26T15:13:12.016+02:00

As can be seen, in the second case, where a ZoneOffset is used, the toString() format omits the square bracket section at the end. By omitting that section, the result is ISO-8601 compatible.

boolean iso8601Compatible = zdt.getZone() instanceof ZoneOffset;

To guarantee an ISO-8601 compatible output use toOffsetDateTime():

String isoCompatible = zdt.toOffsetDateTime().toString();

or a formatter.