Coordinated Universal Time (UTC)
(from the NIST website)
Note: U.S. Government documents are in the public domain and may be freely
distributed so long as content is not changed. This document is being made available for the convenience of RF
How is Coordinated Universal Time (UTC) currently calculated?
There are two ways to think about Coordinated Universal Time (UTC). The way it is usually thought of by most
people is as an indicator of time-of-day (hours, minutes, and seconds). For example, a wall clock can display UTC
in hours, minutes, and seconds. The second way is to think of UTC as a stable frequency or rate which is used to
count seconds. These seconds are then accumulated to form minutes, hours, days, and years. Let's take a brief look
at UTC as a measure of both time-of-day and frequency.
When you use UTC for time-of-day, keep in mind that
it refers to local time at the zero meridian which is near Greenwich, England. The UTC minutes and seconds are
exactly the same as your local time, but the hours are different. The difference in hours between UTC and your
local time depends upon your time zone. For example, when Boulder, Colorado is on Mountain Standard Time, the
difference between Boulder time and UTC is 7 hours (its 7 hours later in England than it is in Boulder). However,
UTC does not observe Daylight Saving Time, and never adds or subtracts an hour. Therefore, when Boulder switches
from Mountain Standard Time to Mountain Daylight Time, the difference between local time and UTC becomes just 6
hours. To get local time from a UTC broadcast, both a time zone and daylight saving time correction usually needs
to be made. Fortunately, these corrections are made automatically by the radio receivers and software packages
that access NIST services after you configure them for your time zone.
The frequency or rate of UTC is
computed by the International Bureau of Weights and Measures (BIPM) located near Paris, France. The BIPM uses a
weighted average from about 250 atomic clocks located in more than 50 national laboratories to construct a time
scale called International Atomic Time (TAI). Once TAI is corrected for leap seconds, it becomes UTC, or the
official world time scale. NIST distributes a real time version of UTC called UTC(NIST) to the public through its
time and frequency services.
How does Global Positioning System (GPS) time differ from Coordinated Universal Time
GPS time differs from UTC by the integer number of leap seconds that have occurred since the GPS time scale
began on January 6, 1980. This difference equaled 13 seconds at the end of 2004.The integer-second difference is
included in the GPS broadcast message, and is usually applied automatically so that GPS clocks display the same
hours, minutes, and seconds as UTC clocks.
GPS time also differs from UTC by a small number of nanoseconds
(nearly always < 25 ns) that continuously changes. The small number of nanoseconds represents the difference
between the GPS time scale on-time marker (OTM) and an estimation of the OTM for the UTC time scale maintained by
the United States Naval Observatory, called UTC(USNO). The current difference between the UTC(USNO) estimate and
GPS time is also part of the GPS broadcast message. GPS timing receivers generally apply this correction to their
1 pulse per second (pps) timing signals, so that the received 1 pps signal represents a real-time estimation of
UTC(USNO). UTC(NIST) and UTC(USNO) are kept in very close agreement, and can be considered equivalent for nearly
Set Your Computer Clock to NIST Time
It's fast and easy to synchronize your computer clock to NIST time ....
You can synchronize your computer
clock with UTC(NIST). There are two easy ways to connect to NIST, as described below:
This is the method of choice for most computer users. The NIST Internet Time Service (ITS) allows you to quickly
synchronize the clock of any computer connected to the Internet. Simple client software allows you to synchronize
your clock as often as necessary, and the service is completely free.
The NIST Automated Computer Time Service (ACTS) allows computers with analog modems to synchronize their clocks by
telephone using simple client software. This service is intended for computers that are not connected to the
Internet, or that are behind a firewall. It requires making a phone call of less than 1 minute each time you set
your clock. The call is long distance outside of the Denver/Boulder, Colorado calling area.
Keep in mind
that ACTS only works with analog modems that use ordinary telephone lines. Digital modems, such as Digital
Subscriber Line (DSL) and cable modems, cannot connect to ACTS. If your computer has a digital modem, use the
Internet Time Service
to synchronize to NIST via your Internet connection.
You can also manually set a clock on time using the
nist.time.gov web site.