I find that storing the time/date in the database is the best way. Then you can use MySQL's nice little functions like DATE_FORMAT and what not.
If all you are doing is storing when the account was created, just use a simple DATETIME field, and in the query, do this:
INSERT INTO table ... VALUES (..., UNIX_TIMESTAMP())
The UNIX_TIMESTAMP() function will insert the UNIX timestamp according to GMT...
That should solve all of your problems!
-Percy