Time zones are areas of the Earth that follow the same definition of time. Formerly, people were using apparent solar time, resulting in the time differing slightly from town to town. Time zones partially rectified the problem by setting the clocks of a region to the same mean solar time. Time zones are generally centered on meridians of a longitude that is a multiple of 15º however as the map below shows, the shapes of time zones can be quite irregular because of boundaries of countries.
All time zones are defined relative to Coordinated Universal Time (UTC). The reference point for Time Zones is the Prime Meridian (longitude 000) which passes through inside the Royal Greenwich Observatory in Greenwich, London, England . The reference time was originally called Greenwich Mean Time (GMT), but is now referred to as Coordinated Universal Time (UTC) to distinguish it from the local time in London which may be adjusted for daylight saving time.
The time for a location is given relative to UTC. Some examples:
- Los Angeles, California, USA: UTC-8 (e.g. if it is 12noon UTC, then it is 4am in Los Angeles)
- New York, New York, USA: UTC-5 (e.g. if it is 11am UTC, then it is 6am in New York)
- Stockholm, Sweden: UTC+1 (e.g. if it is 12noon UTC, then it is 1pm in Stockholm)
- Mumbai, India: UTC+5 (e.g. if it is 1pm UTC, then it is 5pm in New Dehli)
- Tokyo, Japan: UTC+9 (e.g. if it is 11am UTC, then it is 8pm in Tokyo)
- Cairo, Egypt: UTC+2 (e.g. if it is 11pm UTC on Monday 15 March, then the time in Cairo is 1 am, Tuesday 16 March)
- Auckland, New Zealand: UTC+12 (e.g. if it is 9pm UTC on Wednesday 30 June, then the time in Auckland is 9 am, Thursday 1 July)
- Buenos Aires, Argentina: UTC-4 (e.g. if it is 3am UTC on Saturday 23 July, then the time in Buenos Aires is 11 pm, Friday 22 July)
- Honolulu, Hawaii, USA: UTC-10 (e.g. if it is 6am UTC on Monday 1 May, then the time in Honolulu is 8 pm, Sunday 30 April)
- e.g. New Zealand which is usually UTC+12, observes a one hour daylight saving time adjustment during the southern hemisphere summer resulting in a local time of UTC+13!
A map of the world's time zones,
Note that some regions differ 24 hours in time: they have the same time of the day but differ a full day. The two extreme time zones even differ 25 hours, hence during one hour a day the date differs 2 days.
UTC-11 (BEST - Bering Standard Time)
UTC-10 (HST - Hawaii-Aleutian Standard Time)
UTC-9 (AKST - Alaska Standard Time)
UTC-8 (PST - Pacific Standard Time)
UTC-7 (MST - Mountain Standard Time)
UTC-6 (CST - Central Standard Time)
UTC-5 (EST - Eastern Standard Time)
UTC-4 (AST - Atlantic Standard Time)
UTC-3:30 (NST - Newfoundland Standard Time)
UTC (WET - Western European Time, also called GMT)
UTC+1 (CET - Central European Time, also called MET - Middle European Time)
UTC+2 (EET - Eastern European Time)
UTC+3 (MSK - Moscow Time)
UTC+8 (AWST - Australian Western Standard Time)
China has the same time, which makes this time zone exceptionally wide. In the extreme west of China the sun is at its highest at 3 pm, in the extreme east at 11 am.
UTC+9:30 (ACST - Australian Central Standard Time)
UTC+10 (AEST - Australian Eastern Standard Time)