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.

Calculating Local 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:

• 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)

Where the adjustment for time zones results in a time after 12midnight, then the date at the location is one day later. Some examples:

• 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)

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:

• 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)

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 Paris, 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 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 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.

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

• 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 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.

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!

Once the daylight saving time adjustment has been made to the UTC for the location, then the calculations can be performed exactly as given above.

See also: Time zone Daylight saving time