You might want to separate the date and time into 2 fields.
You could use the addseconds function, if you consistently wanted 2 hours.
i.e. addseconds(timefield),7200)
you could also do something like the following addseconds(substring-after(now(),"T"),7200) if you used the now function.
Anything more complicated you'll most likely need other fields so that you can do the math easier.