feat: add period format

[pwmcintyre]

Similar to #718 — this proposal completes the set by adding the period format — as specific in RFC3339 Appendix A

Snippet:

Periods:

   period-explicit   = iso-date-time "/" iso-date-time
   period-start      = iso-date-time "/" duration
   period-end        = duration "/" iso-date-time

   period            = period-explicit / period-start / period-end

— https://datatracker.ietf.org/doc/html/rfc3339#appendix-A

[gregsdennis]

Thanks, but at this time, I don't think we're up for adding onto format. It has a history of poor support and interoperability. Even the tests for validating format are optional.

However, as an annotation keyword (its default mode), you can just put format: period, and most validators will just allow the value. You'll need to actually do the validation yourself in app code.

Beyond that, I think your best bet is to work with the implementation you're using to see what support they have for custom formats.

[awwright]

As a well-defined and useful standard, I think this makes sense to be in core. But like @gregsdennis mentioned, there has been a bit of a change happening in how we approach "format" keywords and extensibility in general, so I filed #1391 to address that aspect.

Is "period" discussed in any other repo? If so, I think that issue plus 1391 will suffice and we can close this.