Comment by wanderingstan

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

Also note that the ISO date standard allows you to drop any digits you aren’t sure about, so all these are valid:

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