As there is a time difference between South Africa and Australia, someone living in South Africa who wants to watch a televised cricket match in Australia will have to get up very early in the morning. A match in Sydney (Australia) may start at 11:00 Sydney time, but in South Africa it is 01:00. Because there is a 10-hour time difference between South Africa and Sydney, and because Sydney is to the EAST of South Africa, and, therefore is AHEAD in time, the match will start 10 hours ahead of South African time.

Similarly, to watch a televised basketball match starting at 14:00 (2 pm) in New York, a fan in South Africa will need to turn on his television at 21:00 (9 pm) that day. Because there is a 7-hour time difference between New York and South Africa, and because New York is to the WEST of South Africa, and therefore is BEHIND in time, the match in New York will start seven hours later in South African time.

While a Japanese shipowner is having a lunchtime meeting on Tuesday in Tokyo, his staff in his New York office will be about to go to sleep on Monday night as it is around 22:00 (10 pm)! Staff members in his San Francisco office will have just had their supper on Monday evening!

Because of these differences in time, shipping is regarded as a 24-hour business, and all involved in shipping need to be aware of the time differences.

Once we have calculated the difference in time in hours, we can calculate the sun time at a given place.

Look at the diagram below :

We can easily calculate that the difference in Longitude is 60˚ and therefore the difference in time is 4 hours. But what is the sun time at Place B? Now follow these steps to find the sun time at Place B:

• Divide the difference in longitude by 15
• Difference in time = 4 hours.
• B is to the EAST of A.
• B is AHEAD of A in time
• We ADD the 4 hours to the known time at A.
• Therefore time at Place B :

10:00 on 23 June + 4 hours = 14:00 on 23 June.

Here are some others :

We can easily calculate that the difference in Longitude is 90˚ and therefore the difference in time is 6 hours. But what is the sun time at Place C? Now follow these steps to find the sun time at Place C:

• Difference in time = 6 hours.
• C is to the WEST of D.
• C is BEHIND D in time.
• We SUBTRACT the 6 hours from the known time at D.
• Therefore time at Place C :

14:00 on 19 June – 6 hours = 08:00 on 19 June.

What is the time at Place N?

We can easily calculate that the difference in Longitude is 225˚ and therefore the difference in time is 15 hours. But what is the sun time at Place N? Now follow these steps to find the sun time at Place N:

• Difference in time = 15 hours.
• N is to the EAST of M.
• N is AHEAD of M in time
• We ADD the 15 hours to the known time at M.
• Therefore time at Place N :

06:00 on 12 June + 15 hours = 21:00 (9 pm) on 12 June.

What is the time at Place Q?

We can easily calculate that the difference in Longitude is 165˚ and therefore the difference in time is 11 hours. But what is the sun time at Place P? Now follow these steps to find the sun time at Place P:

• Difference in time = 11 hours.
• P is to the WEST of Q.
• P is BEHIND Q in time.
• We SUBTRACT the 11 hours from the known time at Q.
• Therefore time at Place P :

0700 on 15 June – 11 hours = 20:00 (8 pm) on day before (14 June.)

Test yourself : Calculating time

1. If the time at Place P (105˚ E) is 14:00 (2 pm) on 19 July, what is the time at each of the following places?

1.1.      Place Q (15˚ E)

1.2.      Place R (135˚ W)

1.3.      Place S (165˚ E)

2. A fire breaks out on board a ship in Port X (165˚E) at 08:30 on 14 May. The ship’s owner has his office at Port Y (75˚W) and has the following office hours : 08:00 to 18:30. He takes about an hour to travel to his office each morning and home again each evening. The ship’s captain tries to telephone the owner on his (the owner’s) mobile telephone number, but it is on voice mail. Should the captain telephone the owner on his office number or on his home number?