Writing calculations that round time requires you to know how FileMaker Pro stores time: As a number of seconds since the beginning of the day. If you use the Data Viewer in FileMaker Advanced and reference a time field via the GetAsNumber function, it displays the number of seconds that have elapsed since 12:00:00 AM. In other words, 12:00:00 AM returns a zero (0) and 12:00:01 AM returns a one (1).
There are 86,400 seconds in one day, 3,600 seconds in one hour, and 900 seconds in a quarter hour. With this knowledge you can round time to any value you want. For example, you might want to round the time entered to the nearest quarter hour. You could use the following formula in an auto-enter calculation to modify any entry, a validation to make sure times are entered in quarter hours only, or anywhere the calculation dialog lets you enter a formula.
Round(MyTimeField / 900; 0) * 900
If you prefer to always round up, use this formula:
Truncate((MyTimeField - 1) / 900; 0) * 900 + 900