Converting time to hours and minutes

I have created a form that will track comp time which is calculated at time and a half. I'd like for the user to be able to put their time in using hours and minutes....for example 2:45 = two hours and 45 minutes. I'd like to be able to multiply this time 1.5 to figure their earned comp time and have it display back in the hours and minutes format....for example 4:08. Does anyone know how to do this? Thanks!!!