The Sun shines on different parts of the Earth at different times. So, while someone in England is having breakfast and watching the sun rinse, someone in India is having their midday meal, someone in New Zealand in the South Pacific is watching the sun set, and it is the middle of the night in the Americas.
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2 Calculating the time difference between two locations 3 Adjusting for Daylight Saving Time |
The local time for a location can be calculated using the time zone for the location. The reference time for time zones was originally called Greenwich Mean Time (GMT), but is now referred to as Coordinated Universal Time (UTC).
The time for a location is given relative to UTC. Some examples:
Calculating Local Time
Where the adjustment for time zones results in a time after 12midnight, then the date at the location is one day later. Some examples:
Where the adjustment for time zones results in a time before 00:00 (i.e. midnight), then the date at the location is one day earlier. Some examples:
Note: The time zone adjustment for a specific location may vary due to the use of daylight saving time.
- 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!
Calculating the time difference between two locations
The time difference between two locations can be determined by subtracting the UTC time adjustment of the current location from the UTC time adjustment for the target location. Some examples:
- You are in Paris, France (UTC+1) and want to know the time in Shanghai, China (UTC+8).
- First subtract your local UTC adjustment (+1) from the adjustment for the target location (+8) giving an adjustment of Local Time +7 (+8 - +1 = +7)
- So, if it is 3pm in London, it will be 10pm in Shanghai.
- You are in Shanghai, China (UTC+8) and want to know the time in Paris, France (UTC+1).
- First subtract your local UTC adjustment (+8) from the adjustment for the target location (+1) giving an adjustment of Local Time -7 (+1 - +8 = -7)
- So, if it is 11am in Shanghai, it will be 4am in London.
- You are in New York, USA (UTC-5) and want to know the time in Rome, Italy (UTC+1).
- First subtract your local UTC adjustment (-5) from the adjustment for the target location (+1) giving an adjustment of Local Time +6 (+1 - -5 = +6)
- So, if it is 3pm in New York, it will be 9pm in Rome.
- You are in Rome, Italy (UTC+1) and want to know the time in New York, USA (UTC-5).
- First subtract your local UTC adjustment (+1) from the adjustment for the target location (-5) giving an adjustment of Local Time -6 (-5 - +1 = -6)
- So, if it is 2pm in Rome, it will be 8am in New York.
- For instance, someone wants to make a business phone call phone call from Washington DC, USA (UTC-5) to Sydney, Australia (UTC+11). The time difference is +16 hours.
- If they try to make this call at 8:30am on Thursday, the time in Sydney would be 2:30am Friday.
- They would be better to wait until 3pm to make the call, when the time in Sydney would be 9am Friday.
- If they waited until 8am Friday in Washinton DC to make the call it might not get answered as it would be Saturday in Sydney.
- For instance: someone calling from Sydney to Washington DC on Monday might not get through as it would still be Sunday in Washington DC.
Adjusting for Daylight Saving Time
Daylight saving time adds one hour to the local time (UTC+1). Some examples:
- Chicago, Illinois, USA, is normally UTC-6, but during the Northern Hemisphere summer it is UTC-5.
- New Zealand in the [South Pacific]] is usually UTC+12, but during the Southern Hemisphere summer it is UTC+13!
See also: Time zone Daylight saving time