How do I add or subtract minutes from an Apointment Start time ?

Hi

Lets say I have 3 types of time durations TypeA-15min, TypeB-30min and TypeC-45min.

I want to add this duration to the calendar time selected so I would have:

If calendar start-time was selected at 7:00am and then I selected TypeB-30min, the end-time would be set/changed to 7:30am

Is this possible?

Thanks

Andreas