Any time i've needed to deal with time specifically i've made 3 ints, seconds, minutes, hours. once an int hit 60 it would add 1 to the next value (however in my game it represented a 9-5 job and things were sped up).
If what you want is to add values i only see the need to make an "int get difference" action, so when one of the ints hits 60, you add one to the next int and store the remainder as the current int.
ie X=60 (seconds), y=current time + z=added time, y+z=more than 60, store difference as a value. If this action spits out a value (on true?), then add 1 to the next value (minutes) but the addition would be a separate action.
The ":" is purely cosmetic.
This may be messy, inefficient or completely useless, but real time isn't measured in delta time and its the only way that made sense to me.
Perhaps even a portion of what i'm suggesting may lead you to a better solution.