Skip to main content
U.S. flag

An official website of the United States government

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

Time and Frequency Frequently Asked Questions (FAQ)

cycle clock illustration

What time is it?
How to get the time using a telephone, computer or radio signals.

NIST Radio Broadcasts
Descriptions and uses of radio signals from WWV, WWVB and WWVH.

NIST time
How is UTC(NIST) related to Coordinated Universal Time (UTC). International Atomic Time (TAI), Greenwich Mean Time (GMT). USNO time, GPS time and Zulu time?

Local Time
Daylight saving time rules, exceptions and legislation. Time zones, borders, abbreviations and relationship to U.S. postal zip codes.

Leap seconds
What leap seconds are, how they are implemented and their future.

Timekeeping and Clocks
How clocks work. Cesium clocks, why we need precise time and frequency. Stopwatch calibrations, calendars and history.

Times of day
How to use a.m./p.m. vs. noon/midnight. Sunrise/sunset times and definitions of dusk/dawn/twilight.

If you have a question that is not addressed here, let us know.

Created September 17, 2009, Updated December 30, 2019