Some various expressions of time that are commonly used.
UTC, Coordinated Universal Time, is the world-wide scientific standard
of timekeeping. It is based upon carefully maintained atomic clocks and
is highly stable.
Its rate does not change by more than about 100 picoseconds per day.
The addition or subtraction of leap seconds, as necessary, at two opportunities
every year
adjusts UTC for irregularities in Earth's rotation.
UTC is used by astronomers, navigators, the Deep Space Network (DSN),
and other scientific disciplines. Its reference point is Greenwich, England:
when it is
midnight there on Earth's prime meridian, it is midnight (00:00:00.000000)
-- "all balls"--UTC. The U.S. Naval Observatory website provides information
in depth
on the derivation of UTC.
UT, Universal Time also called Zulu (Z) time, was previously called
Greenwich Mean Time, GMT. It is based on the imaginary "mean sun," which
averages out the
effects on the length of the solar day caused by Earth's slightly non-circular
orbit about the sun.
UT is not updated with leap seconds as is UTC. Its reference point is also Greenwich, England: when it is noon on the prime meridian, it is noon (12:00:00) UT.
It is common to see outdated references to GMT. It is also common to encounter references to UT or GMT when the system actually in use is UTC.
Local time is UT adjusted for location around the Earth in time zones.
Its reference point is one's immediate locality, when it is 12:00:00 noon
Pacific Time, it is
20:00:00 UTC, and 13:00:00 Mountain Time in Denver, Colorado. Many
locations change between standard time and daylight saving time.
Your Computer thinks it is
Want to check the time? Then use link below.
The official U.S. time
Time information
The United States Naval Observatory
Or this one
NIST Time and
Frequency Division
Are noon and midnight 12 a.m. or 12 p.m.?
This is a tricky question. The answer is that the terms 12 a.m. and 12 p.m. are wrong and should not be used.
To illustrate this, consider that "a.m" and "p.m." are abbreviations for
"ante meridiem" and "post meridiem." They mean "before noon"
and "after noon," respectively. Noon is neither before or after noon; it
is simply noon. Therefore, neither the "a.m." nor "p.m."
designation is correct. On the other hand, midnight is both 12 hours before
noon and 12 hours after noon. Therefore, either 12 a.m.
or 12 p.m. could work as a designation for midnight, but both would be
ambiguous as to the date intended.
When a specific date is important, and when we can use a 24-hour clock,
we prefer to designate that moment not as 1200 midnight,
but rather as 0000 if we are referring to the beginning of a given day
(or date), or 2400 if we are designating the end of a given day
(or date).
To be certain of avoiding ambiguity (while still using a 12-hour clock),
specify an event as beginning at 1201 a.m. or ending at 1159
p.m., for example; this method is used by the railroads and airlines for
schedules, and is often found on legal papers such as contracts
and insurance policies.
If one is referring not to a specific date, but rather to several days,
or days in general, use the terms noon and midnight instead of 12
a.m. and 12 p.m. For example, a bank might be open on Saturdays from 8
a.m. to noon. Or a grocery store might be open daily until
midnight. The terms "12 noon" and "12 midnight" are also correct, though
redundant.
Why is UTC used as the acronym for Coordinated Universal Time instead of CUT?
In 1970 the Coordinated Universal Time system was devised by an international
advisory group of technical experts within the
International Telecommunication Union (ITU). The ITU felt it was best to
designate a single abbreviation for use in all languages in
order to minimize confusion. Since unanimous agreement could not be achieved
on using either the English word order, CUT, or the
French word order, TUC, the acronym UTC was chosen as a compromise.
Does the switch to daylight saving time affect UTC?
No. UTC, which refers to time on the zero or Greenwich meridian, is not
adjusted to reflect changes either to or from Daylight
Saving Time. Universal Time (UT) is the general designation of time scales
based on the rotation of the Earth.
For additional information, please see an exhibit on daylight saving time.
When did the current millennium begin?
Since a millennium is 1000 years, and the first millennium began at the
start of the year 1, it ended at the end of the year 1000. The
second millennium then began with the year 1001 and concluded at the end
of the year 2000. Therefore, the current millennium
technically began with the year 2001. For historical information, see an
exhibit on Calendars.
What is a Modified Julian Date (MJD)?
This is a continuous count of the number of days elapsed since 17 November
1858. It is often more useful than conventional
calendar dates for record keeping over long periods of time, since the
MJD's of two events can easily be subtracted to determine the
time difference in days. Usually, the MJD is specified as a number with
5 significant digits. As an example, the MJD for 1 January
1995 is 49718, meaning that this many days have elapsed between 17 November
1858 and 1 January 1995.
Next to the day, the week is the most important calendric
unit in our life. And yet, there is no astronomical significance to the
week. Nothing
cosmic happens in the heavens in seven days. How, then, did the week
come to assume such importance?
The first thing to understand is that a week is not
necessarily seven days. In pre-literate societies weeks of 4 to 10 days
were observed; those
weeks were typically the interval from one market day to the next.
Four to 10 days gave farmers enough time to accumulate and transport
goods
to sell. (The one week that was almost always avoided was the 7-day
week -- it was considered unlucky!) The 7-day week was introduced in
Rome (where ides, nones, and calends were the vogue) in the first century
A.D. by Persian astrology fanatics, not by Christians or Jews.
The idea was that there would be a day for the five known planets,
plus the sun and the moon, making seven; this was an ancient West Asian
idea. However, when Christianity became the official religion of the
Roman empire in the time of Constantine (c. 325 A.D.), the familiar
Hebrew-Christian week of 7 days, beginning on Sunday, became conflated
with the pagan week and took its place in the Julian calendar.
Thereafter, it seemed to Christians that the week Rome now observed
was seamless with the 7-day week of the Bible -- even though its pagan
roots were obvious in the names of the days: Saturn's day, Sun's day,
Moon's day. The other days take their equally pagan names in English
from a detour into Norse mythology: Tiw's day, Woden's day, Thor's
day, and Fria's day. The amazing thing is that today the 7-day week, which
is widely viewed as being Judeo-Christian, even Bible-based, holds
sway for civil purposes over the entire world, including countries where
Judaism
and Christianity are anathema. Chinese, Arabs, Indians, Africans, Japanese,
and a hundred others sit down at the U.N. to the tune of a 7-day week,
in perfect peace (at least calendrically!).
So dear is this succession of 7 days that when the
calendar changed from Julian to Gregorian the week was preserved, though
not
the days of the month: in 1752, in England, Sept. 14 followed Sept.
2 -- but Thursday followed Wednesday, as always.
Eleven days disappeared from the calendar -- but not from the week!