When I work with dates and times, I prefer to use UNIX Timestamps instead of the MySQL Date datatype.
A Unix timestamp looks like this: 1061265600 for August 9th 2003. Doesn't make much sense, right? It's actually the number of seconds since midnight on January 1st 1970.
You can set your MYSQL fields to regular old INT and store this number as the date.
PHP has a few nifty functions that can turn this number into something useful. The function called time() spits out the current timestamp. Another one is called date().
date("M-d-Y", time()) will spit out today's date formatted as Jul-05-2003. You can find a complete description of the date() function at the PHP website.
The cool thing about Unix Timestamps is that they make date arithmetic extremely easy. To add 6 hours onto an existing time, you just say, $date += 60606;
60606 is the number of seconds in 6 hours.
Once you've done all the math with PHP, you can just use your MySQL Insert and Update queries to fix up your database.
Hope this helps!
-Aaron
