need help calculating time in javascript.

Dear Adobe

I require assistance on a script to calculate a new time of a field. 

This is a navigation form that requires the user to insert a departure time in minutes, then add more time to that in aseparate field and show the result in the next field.

i.e.

Departing at 10:00 (field1)

Time to next waypoint = 78 miutes 

Passing next waypoint = 11:18  (field3)

I need to calculate field3 and display like that, preferably with AM/PM if possible.

Any advice would be appreciated, thanks.