Maybe it's not important to have a super-easy way to get the date only,
in which case a combined date/time string would be fine.

Date/times are currently used for:

* release-date
* build-date
* image-date

Of which release-date is date-only anyway (a "release time" is a bit of
a bogus concept), and for the other two, one can make many builds or
save many images on the same day.