How do I create a time field that's export value is 3 hours ahead of the current time?

I need to create a field on one of our forms that takes the current time from another field and adds 3 hours to it. The JavaScript to pull the current time is:

var tm = this.getField("Time");

if(tm.value == '' || tm == ' ') tm.value =
util.printd("HH:MM tt",new Date());