Calculating local time

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The Sun shines on different parts of the Earth at different times. So, while someone in England is having breakfast and watching the sunrise, someone in India is having their midday meal, someone in New Zealand in the South Pacific is watching the sunset, and it is the middle of the night in the Americas.

[edit] Calculating the time difference between two locations

The following section is only accurate when daylight saving time rules are the same for both locations. Wikipedia is not responsible for errors you make using this guide.

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 Paris, it is 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 is 4am in Paris.
• 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 is 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 is 8am in New York.

Again, where the adjustment crosses midnight, then the date must be adjusted accordingly:

• For instance, someone wants to make a business 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 12:30am Friday.
  • They would be better to wait until 5pm to make the call, when the time in Sydney would be 9am Friday.
  • If they waited until 8am Friday in Washington DC to make the call it might not get answered as it would be Saturday in Sydney.

This works both ways:

• For instance: someone calling from Sydney to Washington DC on Monday might not get through as it would still be Sunday in Washington DC.