Windows Responds With Unacceptable Character

I have a large number of mp3's that have been recorded over the years. In XP, I wanted to give each mp3 a specific 'original recording date'. I used the 'year' field and gave it a date in this format. 2007/05/22. I could then sort properly. In Windows 7, however, when I add 'year' column to the properties and look at the same files, I only see 2007, not the full date. When I try to edit the 'year' field, Windows responds with 'unacceptable character', probably referring to theI can do without the / (20070522), but I do need the full date, not just the year.