NIST (National Institute of Standards and Technology), it seems that the time around the world is all verified from there, and the Windows built-in Internet time is updating from there all the time, too.
NISTIME 32 is an official clock synchronization and calibration applet from the NIST Internet Time Service, witch can be accurate to the 0.01 second at most, and identifies your current time zone immediately when running.
BTW, Windows, Mac and Linux all have built-in tools to sync the system clock automatically and periodically using an NTP (Network Time Protocol) server:
- Windows: Double-click the system clock and then click on the Internet Time tab.
- Mac: Applications > System Preferences > Date & Time
- Linux: System > Admin >Time and Date
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