Script for adding a duration to a time

Is it possible to add a user entered duration to an entered time.

For Example

A field "starttime" entered by a user

A field "enroute" entered by a user

starttime + enroute = newtime

starttime is a time in 24hr format

enroute is a duration, 1h 15m for example, which is variable entered by the user up to 8 hours

So if starttime is 16:24 and enroute is 1:15, newtime would be 17:39