Coordinated Universal Time (UTC)

Coordinated Universal Time (UTC), also referred to as Zulu Time by the U.S. military, and mistakenly (due to different measuring methodologies that result in differences on the order of a second) referred to as Greenwich Mean Time (GMT) (time in Greenwich, England recognized by international conference in 1884 as the International Time), is the time at the prime meridian (0° ± 7.5° longitude).

Hours behind (-), or ahead of, UT

Each time zone is 15° wide (1 hour). The International Date Line is on the other side of the world (from the prime meridian) in the middle of the 12th UTC zone at 180° longitude.
NOTE: The word order of Coordinated Universal Time versus the UTC abbreviation (out of order) is intentional -- established by the Bureau International de l'Heure (International Time Bureau) as a language-independent abbreviation.

Standard Time Zone		Pacific PST		Mountain MST		Central CST		Eastern EST		longitude 0° ± 7.5° UTC, Zulu, GMT
Hours Behind UTC		-8		-7		-6		-5		0

Daylight savings
Time Zone Pacific
PDT Mountain
MDT Central
CDT Eastern
EDT longitude 0° ± 7.5°
UTC, Zulu, GMT

Hours Behind UTC -7 -6 -5 -4 0

Below list local times (in am/pm and 24 hour time) for the 4 time zones in the continental United States versus UTC. Note, 24 hour time refers to midnight as 00:00 hours or 24:00 hours.

COORDINATED    Eastern        Central         Mountain        Pacific     
 UNIVERSAL     Standard       Standard        Standard        Standard   
 TIME (UTC)    Time (EST)     Time (CST)      Time (MST)      Time (PST)

 MIDNIGHT     7pm, 19:00      6pm, 18:00      5pm, 17:00      4pm, 16:00
   01:00      8pm, 20:00      7pm, 19:00      6pm, 18:00      5pm, 17:00
   02:00      9pm, 21:00      8pm, 20:00      7pm, 19:00      6pm, 18:00
   03:00     10pm, 22:00      9pm, 21:00      8pm, 20:00      7pm, 19:00
   04:00     11pm, 23:00     10pm, 22:00      9pm, 21:00      8pm, 20:00
   05:00       MIDNIGHT      11pm, 23:00     10pm, 22:00      9pm, 21:00
   06:00      1am, 01:00       MIDNIGHT      11pm, 23:00     10pm, 22:00
   07:00      2am, 02:00      1am, 01:00       MIDNIGHT      11pm, 23:00
   08:00      3am, 03:00      2am, 02:00      1am, 01:00       MIDNIGHT
   09:00      4am, 04:00      3am, 03:00      2am, 02:00      1am, 01:00
   10:00      5am, 05:00      4am, 04:00      3am, 03:00      2am, 02:00
   11:00      6am, 06:00      5am, 05:00      4am, 04:00      3am, 03:00
NOON 12:00    7am, 07:00      6am, 06:00      5am, 05:00      4am, 04:00
   13:00      8am, 08:00      7am, 07:00      6am, 06:00      5am, 05:00
   14:00      9am, 09:00      8am, 08:00      7am, 07:00      6am, 06:00
   15:00     10am, 10:00      9am, 09:00      8am, 08:00      7am, 07:00
   16:00     11am, 11:00     10am, 10:00      9am, 09:00      8am, 08:00
   17:00     NOON  12:00     11am, 11:00     10am, 10:00      9am, 09:00
   18:00      1pm, 13:00     NOON  12:00     11am, 11:00     10am, 10:00
   19:00      2pm, 14:00      1pm, 13:00     NOON  12:00     11am, 11:00
   20:00      3pm, 15:00      2pm, 14:00      1pm, 13:00     NOON  12:00
   21:00      4pm, 16:00      3pm, 15:00      2pm, 14:00      1pm, 13:00
   22:00      5pm, 17:00      4pm, 16:00      3pm, 15:00      2pm, 14:00
   23:00      6pm, 18:00      5pm, 17:00      4pm, 16:00      3pm, 15:00

Time Zone Chart (PDF file)
8.5 x 14 inches, 240 kB, color

DEFINITIONS

Atomic clock -- usually multiple clocks that count seconds (average of all clocks in system) based on the frequency of the cesium 133 atom (Cs) which oscillates at 9,192,631,770 Hertz.
Mean solar time -- a measurement of the Earth's rotation with respect to the 'fictitious' mean sun.
second -- One second (1 s or 1 sec.) is the duration of 9,192,631,770 periods of radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom (Cs).
UT -- An abbreviation (for Universal Time) that should be used to show approximation of all the below.
UT0 -- mean solar time on the Greenwich meridian obtained from direct astronomical observation. The time 00 hours being at Greenwich mean midnight.
UT1 -- UT0 corrected for polar motion (widely used).
UT2 -- UT1 corrected for seasonal variation in the Earth's rotation (a smoothed scale that does not reflect the real periodic variations in the Earth's angular position).
UTC -- civil atomic time scale on or about the Greenwich meridian kept within 0.9 seconds of UT1.

REFERENCE: A Briefer History of Time -- Howard Barnes, Georgi Dobrovolski Solar Observatory, New Zealand, 2001. (PDF version)

Standard Date Format

Virtually the entire world formats time as Hours:Minutes:Seconds (hours listed before minutes, minutes listed before seconds). The date format Year-Month-Day has become widely accepted as a standard in many countries.
International Standard ISO 8601, United States ANSI X3.30 and NIST FIPS 4-1 standards, Canadian standard CSA Z234.5, European standard EN 28601, British standard BS EN 28601.

ISO 8601 FORMAT    EXAMPLEs

           year    2010
     year-month    2010-09       2010-September
 year-month-day    2010-09-18    2010 September 18    20100918

Year 2000

The United States and many other countries use the Christian (Gregorian) calendar to mark years. This calendar has become a standard in such worldwide industries as aviation, and international commerce and trade -- however, other calendars do exist.

       CALENDAR    YEAR

         Jewish    5760-5761
        Chinese    4698 (Year of the Dragon)
      Old Roman    2753
       Buddhist    2544
      Christian    2000
         Coptic    1716
        Islamic    1420-1421

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Coordinated Universal Time (UTC)

Daylight savings Time Zone		Pacific PDT		Mountain MDT		Central CDT		Eastern EDT		longitude 0° ± 7.5° UTC, Zulu, GMT
Hours Behind UTC		-7		-6		-5		-4		0