New Time Zone Patterns

This proposal was reviewed in the CLDR TC meetings on 2013-01-09 and 2013-01-16, and approved by the CLDR TC.

Summary

This design proposal includes following new pattern letters in the LDML date format pattern definition for time zone formatting.

• 1. X and x for ISO 8601 style non localizable UTC offset format

     1. 'X' uses UTC designator "Z" when UTC offset is 0

     2. 'x' uses difference between local time and UTC always - i.e. format like "+0000" is used when UTC offset is 0.

  2. O for localized GMT format variations

  3. V for time zone ID (V - short / VV - IANA) and exemplar city (VVV).

Background

JDK and LDML started from the same base, but they have been extended independently. LDML is not necessarily 100% compatible with JDK or vise versa, but using completely different definitions for the same purpose introduce confusion among consumers. Now, Java folks are trying to integrate JSR-310 into Java 8 release and extend some date format pattern definitions to support things missing in the current SimpleDateFormat.

For example, LDML specification uses pattern letter 'Z' for UTC offset based time zone format. In CLDR 22.1, letter 'Z' is defined as below:

However, JDK supports ISO8601 style format using letter 'X'. JDK also introduced letter 'I' for direct use of zone ID.

This proposal is created for sorting out time zone patterns and hopefully filling a gap between LDML specification and future JDK and JSR-310 releases, as well as adding necessary pattern for CLDR requirements.

Non-Localized Local Time Offset Format

The pattern letter 'X' was added in JDK 7 for supporting ISO8601 style time zone format. In JDK 7, the behavior of pattern letter 'X' is described as below:

ISO8601TimeZone: OneLetterISO8601TimeZone TwoLetterISO8601TimeZone ThreeLetterISO8601TimeZone OneLetterISO8601TimeZone: Sign TwoDigitHours Z TwoLetterISO8601TimeZone: Sign TwoDigitHours Minutes Z ThreeLetterISO8601TimeZone: Sign TwoDigitHours : Minutes Z

TwoDigitHours: Digit Digit

Sign: one of + - Hours: Digit Digit Digit Minutes: Digit Digit Digit: one of 0 1 2 3 4 5 6 7 8 9

In the current LDML specification, ISO8601 style format is specified by pattern "ZZZZZ" (5 'Z's), but followings are not supported

• • Offset without separator, such as -0800

  • Hour only format, such as -08

The JSR-310 proposal is trying to extend the definition to support seconds field in offset, probably because it is necessary to handle LMT in the time zone database. So the proposal adds "XXXX" and "XXXXX" for supporting optional second field.

Note: ISO8601 specification does not support seconds field in local time offset.

In LDML, we could define "ZZZZZZ" (6 'Z's), "ZZZZZZZ" (7 'Z's)... to support these requirements, but it would become so messy. Luckily, pattern letter 'X' is not yet used by the LDML specification, I propose to use the letter for supporting these requirements and deprecate "ZZZZZ" (5 'Z's).

The JSR-310 definition (compatible with JDK 7 SimpleDateFormat, with some enhancements for seconds field) might be used also for LDML, but I think there are several issues.

• • Single 'X' is used for limiting offset to be hour field only. Such usage is practically questionable. There are some active time zones using offsets with non-zero minutes field. So such format is highly discouraged when a zone has non-zero minutes field. ISO8601 specification also says "The minutes time element of the difference may only be omitted if the difference between the time scales is exactly an integral number of hours.".

  • When non-zero minutes (or seconds) field is truncated and hour field is 0, the output becomes +00/-00/+0000/-0000/+00:00/-00:00. Use of negative sign for offset equivalent to UTC (-00/-0000/-00:00) is illegal in ISO8601.

When to use "Z" or "+00"/"+0000"/"+00:00" is also a design question. JSR-310 seems to extend pattern letter Z to support format without ISO8601 UTC indicator "Z".

In ISO8601 specification, "Z" is specifically defined for UTC of day. "+00"/"+0000"/"+00:00" is a valid format for difference between local time and UTC (Section 4.2.5.1). Semantically, "Z" expresses UTC itself, while "+00"/"+0000"/"+00:00" expresses a local time with UTC offset of zero. LDML specification might handle zone "Etc/UTC" different from winter time of "Europe/London" (the former is formatted as "Z" and the latter is formatted as "+00"/"+0000"/"+00:00"). However, it is not clear how "Etc/GMT", "Etc/GMT+0"... should be handled. (Etc/GMT might be just an alias of Etc/UTC, but Etc/GMT+0 is probably not in this scope...). It would be much easier to understand to control the behavior through different pattern letter.

For this reason, this proposal defines two pattern letter - 'X' and 'x'. 'X' is mostly upward compatible with JDK 7 SimpleDateFormat's 'X' and the current JSR-310 proposal, but slightly modified for resolving some design issues. 'x' only differs that the format always use difference between local time and UTC ("+00"/"+0000"/"+00:00" when UTC offset is 0).

Proposed Definition