Hourglass - Hourglass is the most advanced simple countdown timer for Windows.
NetTime - nettime is a time & attendance software solution.
wnr - Better than pomodoro, this timer app balances work and rest.
Meinberg NTP - NTP client and server download for different operating systems.
Free Countdown Timer - Free Countdown Timer is a free, full-featured and user-friendly countdown timer for Windows
Atomic Clock Sync - download the free atomic clock sync config utility to optimize how your Windows pc receives updates from time servers