It is currently possible to serialize dates as ISO8601DateOnly and ISO8601.

https://gist.github.com/georgehemmings/42d9841913d1da01e91b

The date handler applies to all DateTime fields in the object.

Where a DateTime field is representing only a date, it would be nice to emit this in ISO8601DateOnly but leaving any other DateTimes to emit in full ISO8601 format.