Adding Time

I have a textbox with a time as Now function, how can I make sure if someone wants to alter the time (as it could be a start tiem) that it is entered as hh:mm:ss and allows nothing else?
Also, if I have another text box that has an amount of time in seconds (e.g. 7200), how can I add the start time to this amount and show the new time? e.g. Start time = 15:30:10, 7200 secs pass, finish time = 17:30:10?

DateAdd("s", 7200, "15:30:10")

To ensure correct format for date entry, I would use three textboxes or labels associated to updown controls, or a chencking procedure on keypress or change events...
For adding, rpely has been given !