TimeLeft - TimeLeft is a versatile desktop utility - it can be used as a countdown clock, reminder, clock...
NetTime - nettime is a time & attendance software solution.
Hourglass - Hourglass is the most advanced simple countdown timer for Windows.
Meinberg NTP - NTP client and server download for different operating systems.
wnr - Better than pomodoro, this timer app balances work and rest.
Atomic Clock Sync - download the free atomic clock sync config utility to optimize how your Windows pc receives updates from time servers