The program uses a time server to compare the time on your computer to the 'correct' time. If the time is different by more than a specified period (a few seconds), you'll see a cross in the box on the right-hand side. This doesn't affect the way this tool works, but time synchronisation can be important to certain systems in computing grids. If some action is refused and your system clock is not in time, this could be the problem.
It's recommended that you synchronise your clock with an internet time server following the Network Time Protocol (NTP).