Set up a column in your database that has a function of datetime. Just like you would declare a field with varchar, char, or int, etc. Datatime is the way mysql saves times and dates and that is all you put in this column. You don't need the three fields anymore to save your dates and times.
Then you just set a variable to what you are trying to save such as.
$regdate=date("Y-m-d H:i:s");
The date function here puts into your database the year month day hours and seconds. You can save just the date or time or whatever by putting in the different letters in the date function. After that mysql has functions to perform anything you want on the dates in your datetime field.
You can read more at http://www.mysql.com or get one of the good mysql books such as SAMS learn mysql in three weeks. All the functions are in there.
Hope this helps.
