In mysql v4, there are additional date and time functions such as SUBTIME and TIMEDIFF. What's the difference between the two? From what i've read in the mysql documentation manual, they pertain to the same thing.

I was planning on using it in my query for computing employee lates. For example, an employee's shift is from 7am - 4pm and he comes in at 7:30am. I would like to use either of the time functions to compute the number of minutes late. Same goes for overtime and undertime.