You can have a var called "seconds" or something like this. Every frame you add the delta time of the frame (that is time_frame / 16).
Once you need to display the minutes and seconds, its pretty simple math:
var passedMinutes = seconds / 60;
var passedSeconds = seconds % 60;
Edit: Just found "total_secs" in the manual. You can use this variable instead of counting your own variable up. However, if you want to be able to restart the game, you need to use your own variable so you can set it back any time.
