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.

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.