To make a playlist of "recent additions", I wanted a filter to return all files under my Music folder created after a specified date, regardless of location.
The information field %_file_create_datetime_raw% is available, but there is no way within Mp3tag to get a user-specified timestamp to filter on. The best you can do is to copy an existing timestamp from a file in a custom column. This is awkward and imprecise because you cannot choose the exact date(s) that you want.
Raw timestamps use epoch time (sometimes called Unix time). That is the total number of seconds that have elapsed since January 1, 1970 at midnight GMT, and is the format displayed by both %_file_create_datetime_raw% and %_file_mod_datetime_raw%.
I found a site that makes the conversion easy: www.epochconverter.com You simply enter a normal date in one of their "human date to timestamp" sections, and one click later you have a timestamp value in epoch format. Just paste that number into your filter expression. No need to customize Mp3tag columns.
For example, this filter returns files created after 01 Jun 2012 at 00:00:00 GMT:%_file_create_datetime_raw% GREATER 1338508800
The next filter returns files created between the above date and 16 Jun 2012 00:00:00 GMT:%_file_create_datetime_raw% GREATER 1338422400 AND %_file_create_datetime_raw% LESS 1339804800