I figure out the most significant value I can and just zero/one out the rest. So "February 1976" becomes 1976-02-01 00:00:00. That is an unlikely timestamp, so it tells me the whole thing is an estimate.

This convention works for me because I'm dating slides and negatives taken by deceased relatives before I was born, mostly of children I've only known as adults. Aside from birthday parties I never know the specific day, and it's unlikely anyone in the future will really care beyond having the pictures in a reasonable overall order.