Time Zone Calculations

4.17 - Be able to use data related to time zones

Time Zones vary in width but are broadly 15° with varying distances.

Moving to the East and into another timezone means adding an hour. Moving to the next time zone to the west results in subtracting one hour.

Questions may involve asking for a calculation involving the time at any one moment or calculating time zone based on degree difference.

If asked about a location a number of degrees away remember the Earth can be represented as a circle which is 360°. We calculate positions up to 180° West and East with London set as 0°. If a location is 90° away that is 1/4 of 360. 1/4 of 24 hours is 6 hours so it is likely (but not always) that the other location is 6 timezones away.

Question 1
When it is Noon (12pm) in London (0° Longitude), what time is it in New York at approximately 75°W?.

15 degrees = 1 hour difference
75 divide 15 = 5
5 hours difference. 5 hours west = earlier than London
12pm minus 5 hours = 7am

Question 2

When it is Noon (12pm) in London, it is 10pm in Sydney, Australia.

Dan in London rings his friend Boski in Sydney at 15.30pm on Christmas Day. What time is it in Sydney?

Time difference = 10 hours. 15.30 + 10 hours is 25.3.
There is no 25.3 hours as we measure time. 25.3 minus 24 = 1.3. Correct time is 1.30 am on 26th December

Question 3

San Francisco is approximately 120 degrees west of London. An observer in London sees a clock at 15.30. How long will it be before a clock in San Francisco displays the same time?

360 = 24 hours
360 / 120 = 3
24 hours / 3 = 8
Time difference = 8
The clock would display the same time 8 hours later

Question 4

At 1am, Joanne in Brisbane, Australia (10 hours ahead of London) rings her friend Mary in Hawaii (10 hours behind London) to wish her a happy new year. "What are you ringing me at this time for?" Mary exclaims. When does Mary receive the call?

This crosses the International Date Line so the date will change
Difference is twenty hours
01.00 minus 20 hours = 5.30am on 31st December (the previous day and year)
Mary is confused as to why both her and Joanne are not in Lancashire


  • Longitude = the measurement east or west of the prime meridian
  • 4 minutes = 1 degree of longitude, 1 hour = 15 degrees of longitude