I think this is a bug, albeit a strange one.
I'm trying to change the year tag of the file  from 1941 to 1941-03-26. Since the file is an MP3, this format is ok, conforming to the YYYY-MM-DD specification of the TDRC field in the ID3v2 tag.
In Mp3Tag, the file is changed and I have no problems. When I click save the pop up with "Saving 1 tag of 1 files" appears.
The problem is, no other program reads the 1941-03-26 on the year file. Winamp, iTunes, MediaMonkey, etc. To see how it was a did a small ruby script (with taglib ) to check the year tags, and confirmed it that the date tag was indeed 1941.
I can't understand why Mp3tag but doesn't change the date.
I'm using the 2.57 version.