just to be clear, the native DATETIME format for mySQL is YYYY-MM-DD HH:MM:SS. mySQL also supports it's own TIMESTAMP type (note that this is not a UNIX timestamp). here's where is gets really tricky: prior to mySQL 4.1, the TIMESTAMP format was YYYYMMDDHHMMSS. After mySQL 4.1, the TIMESTAMP format is the same as DATETIME. the TIMESTAMP type is an easy way a recording record INSERT and UPDATE times.
anyway, all of these formats can be easily created using PHP's [man]date[/man] function or with mySQL's DATE_FORMAT function.