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Mountain time

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noun
  1. See under standard time.
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Origin of Mountain time

1880–85; Americanism
Also called Mountain Standard Time.

standard time

noun
  1. the civil time officially adopted for a country or region, usually the civil time of some specific meridian lying within the region. The standard time zones in the U.S. (Atlantic time, Eastern time, Central time, Mountain time, Pacific time, Yukon time, Alaska-Hawaii time, and Bering time) use the civil times of the 60th, 75th, 90th, 105th, 120th, 135th, 150th, and 165th meridians respectively, the difference of time between one zone and the next being exactly one hour.
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Origin of standard time

An Americanism dating back to 1880–85
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

British Dictionary definitions for mountain time

standard time

noun
  1. the official local time of a region or country determined by the distance from Greenwich of a line of longitude passing through the area
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Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

mountain time in Science

standard time

  1. The time in any of the 24 time zones into which the Earth's surface is divided, usually the mean time at the central meridian of the given zone. There are four standard time zones in the contiguous continental United States: Eastern, using the 75th meridian; Central, using the 90th meridian; Mountain, using the 105th meridian; and Pacific, using the 120th meridian. Alaska Standard Time, centered on the 135th meridian, is one hour behind Pacific time, and Hawaii Standard Time, centered on the 150th meridian, is one hour behind Alaska time. See more at daylight-saving time time zone.
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The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.