dont know if this helps or not but it seems a problem of trying to jam a square peg in a round hole =)
from http://www.mysql.com/doc/D/A/DATETIME.html
6.2.2.2 The DATETIME, DATE, and TIMESTAMP Types
The DATETIME, DATE, and TIMESTAMP types are related. This section describes their characteristics, how they are similar, and how they differ.
The DATETIME type is used when you need values that contain both date and time information. MySQL retrieves and displays DATETIME values in 'YYYY-MM-DD HH:MM:SS' format. The supported range is '1000-01-01 00:00:00' to '9999-12-31 23:59:59'. (``Supported'' means that although earlier values might work, there is no guarantee that they will.)
The DATE type is used when you need only a date value, without a time part. MySQL retrieves and displays DATE values in 'YYYY-MM-DD' format. The supported range is '1000-01-01' to '9999-12-31'.
The TIMESTAMP column type provides a type that you can use to automatically mark INSERT or UPDATE operations with the current date and time. If you have multiple TIMESTAMP columns, only the first one is updated automatically.
Automatic updating of the first TIMESTAMP column occurs under any of the following conditions:
The column is not specified explicitly in an INSERT or LOAD DATA INFILE statement.
The column is not specified explicitly in an UPDATE statement and some other column changes value. (Note that an UPDATE that sets a column to the value it already has will not cause the TIMESTAMP column to be updated, because if you set a column to its current value, MySQL ignores the update for efficiency.)
You explicitly set the TIMESTAMP column to NULL.