North American Overlay Mapping
Layer Notes - Time Zones
As with other zone overlays, the Time Zones use different colors on land, and lines in ocean or sea areas, to differentiate between them. They are identified by numbers within or adjacent to each zone. The base-point in any map of time-zones is Longitude 000°, a line running from the North Pole to the South Pole, and passing through the London borough of Greenwich, which has given its name to Greenwich Mean Time, more commonly known these days as 'UTC'. Time zones are all marked with a number relative to the zone at Greenwich, and start either with a '-', indicating that the hour-value is subtracted from the current time at Greenwich, or with a '+', or with no preceeding sign, indicating that the hour-value is added to the current time at Greenwich.
Two examples of this might be helpful: Brazil, is in the '-3' time-zone. If the time in London is 9am, you would subtract the 3 to give a current time of 6am in Brazil. China is in the '+8' time-zone. If the time in London is 9am, you would add the 8 to give a current time of 5pm in China.
Note that no account is taken of any shift caused by Summer Time, when the clocks are moved 1 hour forward in the spring, and 1 hour back in the Autumn. The adoption of Summer Time is neither universal or comprehensively coordinated.
Although the general position of time zones is widely acknowledged, when the scale of a map is increased to the level present in some of the overlays here, there can be some dispute as to the exact border of each zone. I have taken my data from the normal standard sources, though as with other data, no claim is made to the accuracy or even the existence of any time-zone boundary on these maps.