Sir Sanford Fleming proposed dividing the world into 24 time zones spaced 15 degrees of longitude apart in 1878, as the earth rotates once every 24 hours, rotating 15 degrees per hour. Greenwich Mean Time is based on the Greenwich Meridian located at 0 degrees longitude in London, with times east being later and west being earlier. The International Date Line runs through the Pacific Ocean near 180 degrees longitude, requiring the addition or subtraction of a day when crossing it westward or eastward respectively.

1. WORLD WIDE TIME ZONES
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2. WORLD WIDE TIME ZONES
 In 1878, Canadian Sir Sanford Fleming proposed the system of worldwide time zones that we use
today.
 He recommended that the world be divided in to twenty four time zones, each spaced 15 degrees of
longitude apart.
 Since the earth rotates once every 24 hours and there are 360 degrees of longitude, each hour the
earth rotates one twenty fourth of a circle or 15 degree of longitude.

3. GMT (GREENWICH MEAN TIME)
 Greenwich Mean Time (GMT) is the time measured on the Earth's zero degree line of longitude, or
meridian.
 This runs from the North Pole to the South Pole, passing through the Old Royal Observatory in the
London suburb of Greenwich.

4.  This line has been called the Greenwich Meridian since 1884, and it is from here that all terrestrial
longitudes are measured and the world's time zones are calculated.
 Generally, if you are in a country east of the Greenwich Meridian, your local time is ahead of GMT (e.g.
local time in China is GMT +8 hours).
 West of the Greenwich Meridian, local time is behind GMT (e.g. local time in New York is GMT -5 hours
in winter and GMT -4 hours in summer).

12. THE INTERNATIONAL DATE LINE (IDL)
 The International Date Line (IDL) is an imaginary line on Earth's surface located at about 180 ̊ East and
West of the GMT/Prime Meridian.
 The International Date Line runs down the middle of the Pacific Ocean. If you cross the date
line moving east, you subtract a day, whereas if you are moving west you add a day.
 For example, if today is Friday and we crossed the International Date line from west to east then it
would be Thursday.