I worked on a family archiving product (which dead ended) and dealt with this problem. My hacked solution was to repurpose the seconds value to encode precision, as you generally don’t care about seconds.

I forget the exact scheme, but it was something like this:

    0=minute precision 
    1=hour
    2=day
    3=month
    4=quarter/season
    5=year
    6=decade

    1980-12-25T08
    1960-12-06
    1959-12
    196

The last one seems like it could also be problematic, how do you distinguish the 1960s decade from the literal year 196. In this case it’s kind of obvious since photography wasn’t very popular in the year 196.