Basic constraints for month and day value ranges in date and time-of-day/seconds: +1
Standalone relative dates don't strike me as something particularly worthy of providing, at first glance.
I also think that the forced sign looks rather awkward for negative rel-dates: -1-1-1, -10-0-1. Further, I think that date arithmetic with relative dates may be _really_ awkward.
So I'd suggest limiting any-data-time to just abs-date-time or abs-date or rel-time, for starters.
Thanks Andreas! Good feedback. I agree that negative rel-dates aren't pretty. When I wrote examples for my test parser, I used 4 digits for all years, which made them less ugly (-0000-01-00). We could easily require a minimum of 4 digits. Still not perfect, but perhaps better. I also agree that date math is tricky (all around), but Ren doesn't have to do the math. :-) For me, it's a question of "how else do we do it?" if relative dates are useful?
Tough call, as I think there *is* value in relative dates. Some of that value comes from them being imprecise, which conveys meaning as humans sometimes do.