Completing one's Salaah a few seconds before the time expires

One has an atomic wrist-watch. That is a watch that runs exactly on the second with the atomic clock setting. If sunrise on the prayer calendar is at 7.00 and one finishes Fajr salaah at 6.59 and 58 seconds i.e. 2 seconds before 7.00 o'clock. Is the Fajr salaah valid? And what if it was finished 1 second before 7.00 o'clock?