As in JavaScript 1.2, date is measured in milliseconds since midnight 01 January, 1970 UTC. One day is 86,400,000 milliseconds. The Date object can hold a date that is -100,000,000 days to 100,000,000 days relative to 01 January, 1970 UTC. JavaScript 1.3 adds the UTC method to the Date constructor. Date.UTC is different than the Date constructor in that it uses the UTC instead of the local time. Another difference is that Date.UTC returns a numeric value that is equal to the number of milliseconds since midnight 01 January, 1970 UTC. The Date constructor, on the other hand, creates a Date object.

The whole subject of Coordinated Universal Time is fascinating. There are 7 Universal Times (all within 1 second of each other), and UTC is the "coordinated version of 'Universal Time'", hence the word order of Coordinated Universal Time. The abbreviation UTC is a language-independent international abbreviation, it is neither English nor French. It means both 'Coordinated Universal Time' and 'Temps Universel Coordonn?'.

UTC isn't the same as GMT (proper GMT was originally measured from Greenwich mean mid-day, not mid-night [proof: Astronomical Almanacs 1984 onwards, page B5]), as UTC is an ATOMIC time-scale, while GMT (strictly speaking UT1 [UT-one]) is tied to the rotation of the Earth in respect to the fictitious 'mean Sun'. UTC is, however, kept within 0.9 seconds of UT1, by virtue of leap seconds.

To read more on UTC, please visit these references:

BIPM Time Section; the world's GOVERNING BODY for civil atomic time-scales.
BIPM Time Section
The International Earth Rotation Service
The Royal Observatory Greenwich
US. National Institute of Standards & Technology)
UK's National Physical Laboratory
US Navy
US Navy FAQ
US Naval Observatory
The International Telecommunication Union
A Briefer History of Time 

This tip has been contributed by Howard Barnes. He has 23 years of casual research into time-keeping behind him, and was taught by personnel of the old Bureau International de l'Heure [BIH, The International Time Bureau].

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