If the just day is unknown, I use: 1
If I only know the year: 12/31 (a good flag..How many things are completed/released on the last day of the year?

)
Date columns seem to have a variety of data entry modes, some restrictions are not totally obvious. You can Select the entire field, but not work with that. You must work only on 1 date section at a time MM/DD/YYYY