MySQL storing duration time - datatype?

Storing it as an integer number of seconds will be the best way to go.

• The UPDATE will be clean and simple - i.e. duration = duration + $increment
• As Tristram noted, there are limitations to using the TIME field - e.g. "TIME values may range from '-838:59:59' to '838:59:59'"
• The days/hours/minutes/seconds display formatting won't be hardcoded.
• The execution of your other calculations will almost surely be clearer when working with an integer "number of seconds" field.