If I understand everything correctly Instant::now() is not a const function (and I think can't be, since it may return a different value on every call), so it couldn't be used with Field Defaults as specified in the linked draft RFC.
I personally am a fan of the idea that Default values are invariant across runs of the program etc. I am uncomfortable that there is a default for RandomState, although I understand the use in hashing and don't have a great alternative to suggest.