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KF5JRV > TECH 01.04.16 13:42l 78 Lines 4410 Bytes #999 (0) @ USA
BID : 728_KF5JRV
Read: GUEST
Subj: UTC Time
Path: IW8PGT<CX2SA<N9PMO<NS2B<N0KFQ<KF5JRV
Sent: 160401/1117Z 728@KF5JRV.#NWAR.AR.USA.NA BPQ1.4.65
Coordinated Universal Time (UTC)
The ensemble and primary standard form what is known as the
NIST time scale. This time scale produces a very stable and
accurate frequency by using a weighted average of all its
oscillators, with the best oscillators receiving the most weight.
Small adjustments, never more than about 2 nanoseconds per day, are
made to the NIST time scale to keep it in agreement with international
standards. The output of the time scale is called UTC(NIST), which is
short for Coordinated Universal Time kept at NIST.
You can think of UTC(NIST) as both a frequency and a time
standard. It produces an extremely stable frequency that serves as
the standard for the United States. It also produces the standard for
time interval, by generating pulses that occur once per second. By
counting these second pulses, NIST can keep time. The second
pulses are added together to keep track of longer units of time
interval—such as years, months, days, hours, and minutes.
The UTC system of timekeeping is similar to your local time, with
two major differences.
Since UTC is used internationally, it ignores local conventions
such as Daylight Saving Time and time zones. In other words, UTC is
the same no matter where you are located on Earth. Unlike local time,
which is usually based on a 12-hour clock, UTC is a 24-hour clock
system. The hours are numbered from 0 to 23. The time at midnight
is 0 hours, 0 minutes, and 0 seconds. The time just before the next
midnight is 23 hours, 59 minutes, and 59 seconds.
To convert UTC to local time, you need to add or subtract a specific
number of hours. The number of hours to add or subtract depends
on the number of time zones between your location and the zero
meridian that passes through Greenwich, England. When local time
changes from Daylight Saving to Standard Time, or vice versa, UTC does not
change. However, the difference between UTC and local time changes by 1
hour. For example, in New York City, the difference between UTC and local
time is 5 hours when Standard Time is in effect, and 4 hours when Daylight
Saving Time is in effect.
Most of the hardware and software products that access NIST
services allow you to select your time zone and are capable of
automatically converting UTC to your local time. These products also
automatically correct for Daylight Saving Time. The conversion is fair-
ly simple.
Leap Seconds
As we mentioned earlier, the second is defined according to
the intrinsic properties of the cesium atom. This means that UTC is
an atomic time scale , which runs at an almost perfectly constant
rate. Prior to atomic time, time was kept using astronomical
time scales that used the rotation of the Earth as their reference.
When the switch to atomic time keeping occurred, it became obvious
that while much was gained, some things were lost. A few people
still needed time referenced to the Earth’s rotation for
applications such as celestial navigation, satellite observations of
the Earth, and some types of surveying. These applications relied on an
astronomical time scale named UT1. For these reasons, it was agreed
that UTC should never differ from UT1 by more than 0.9 s. Therefore,
those who needed UT1 could just use UTC, since they could be sure that
the difference between the two time scales would be less than 1 s.
Keeping the two time scales in agreement requires making occasional
1 s adjustments to UTC. These adjustments are called
leap seconds.
A leap second can be positive or negative, but so far,
only positive leap seconds have been needed. Leap seconds are
announced by the International Earth Rotation Service and are
usually inserted into the UTC time scale on June 30 or December 31,
making those months 1 s longer than usual. Currently, about 4
leap seconds are required every 5 years.
All NIST services automatically add leap seconds when
necessary. For the very few people who need to know UT1 with
an uncertainty of less than 1 s, most NIST services also broadcast
a UT1 correction . This correction reports the current time
difference between UTC and UT1 to the nearest 0.1 s.
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