Is there a proper way to enter incomplete date information? I've got scans of old images and no way to come up with full dates on them. For example I've got some slides that I know were shot in 1968, but only a guess on the month and no idea about the day. I'd like to be able to add incomplete date information so that they could be found in a search for images from 1968. Is there a right (or at least commonly agreed upon) way to enter a year with unknown month and day? Something like 1968-00-00? Obviously I could just pick a day to be the "unknown date day" but I was wondering if there was a better way or if there was an agreement to use a particular day for that purpose.