# 12.03 International time zones

Lesson

## Time zones

The world has $25$25 standard time zones, ranging from $-12$12 to $+12$+12, plus non-standard time zones, including $+13$+13 and $+14$+14, as well as about $15$15 different half-hour and $45$45-minute offset time zones. A time zone is a region of the Earth that has adopted the same standard time, and those regions are roughly divided up by the lines of longitude. These time zones are based on Greenwich Mean Time (GMT) or Universal Time Coordinated (UTC).

In the following map, the colour of the region is used to indicate the time zone.

Looking at the coloured bar along the bottom of the map, we can see how the time zones should be broken up according to the lines of longitude. Comparing this to the colours of the map, notice that the countries that follow a particular time zone aren't always perfectly within those lines of longitude. There are various reasons for this:

• some countries want to stay in sync with their neighbours' time zones, like many of the countries in Western Europe
• many larger countries, like the USA, break themselves into multiple time zones, so that everyone experiences a similar day
• some countries, even though they are wide and cross many time zones, choose to stay with the same time zone, like China
• some countries and states, like India and Nepal, and South Australia and Northern Territory, use half hourly or quarter hourly time zones, which are indicated with stripes

## Greenwich Mean Time (GMT) and Coordinated Universal Time (UTC)

Greenwich Mean Time (GMT) is a time zone based upon the Prime Meridian, which goes through the Royal Observatory in Greenwich, England. Coordinated Universal Time, or Universal Time Coordinated (UTC), is a time standard, or a system of time-keeping, and is often used interchangeably, as the actual times are the same.

All other time zones are based on how far they are behind or ahead of GMT/UTC. The zone of Greenwich Mean Time is considered to be $0$0. Countries to the west of the Prime Meridian are behind GMT and use a negative ($-$) GMT/UTC. Countries to the East of the Prime Meridian are ahead of GMT and use a positive ($+$+) GMT/UTC.

For example, Argentina and Paraguay are $-3$3 GMT, which means these countries are $3$3 hours behind Greenwich Mean time. So if was midday in England, it would be $9$9 a.m. in Paraguay.

Remember!

As we move west, then the time decreases. Time zones to the west of GMT/UTC use a negative sign ($-$).

As we move east, then the time increases. Time zones to the east of GMT/UTC use a positive sign ($+$+).

## The International Date Line

On the opposite side of the world to the Prime Meridian is the International Date Line. The International Date Line marks the beginning and end of a calendar day ($24$24 hrs). It passes through the middle of the Pacific Ocean at about $180°$180° line of longitude, but it has been drawn around some territories and island groups, just so that the date line wouldn't pass through the middle of a country or town.

(Worldatlas.com)

Countries to the left of the International Date Line, that is countries in $+12$+12 GMT/UTC, spend most of the time ($23$23 hours) a whole day ahead of countries to the right of the International Date Line, that is countries in $-11$11 GMT/UTC. Countries with $+14$+14 GMT/UTC are more than a day ahead of some of their neighbours.

When travelling across the International Date Line, you lose or gain a day depending on the direction you travel.

## Calculating time differences

Using maps like the one above, tables like the one below, or websites like www.timeanddate.com, we can calculate the time difference between two cities or counties

GMT offset Location
$-5$5 New York, Cuba, Peru
$-4$4 Chile, Barbados, Brazil (west), Bolivia
$-3$3 Argentina, Brazil (east), Uruguay, Greenland
$-2$2 South Sandwich Islands
$-1$1 Azores
$0$0 UK, Ireland, Iceland, Portugal, Ghana, Liberia, Mali
$+1$+1 Algeria, Angola, Chad, Bosnia, Croatia
$+2$+2 Finland, Greece, Lebanon, Egypt, South Africa
$+3$+3 Russia (west), Saudi Arabia, Kenya, Madagascar, Iraq
$+3.5$+3.5 Iran
$+4$+4 Mauritius, United Arab Emirates, Armenia
$+4.5$+4.5 Afghanistan
$+5$+5 Pakistan, Maldives, Kazakhstan
$+5.5$+5.5 India, Sri Lanka

From the map above, we can see that Johannesburg is behind Tokyo. Johannesburg is $+2$+2 GMT, and Tokyo is $+9$+9 GMT, which means the difference is $+9-+2=7$+9+2=7 hours. This means Johannesburg is $7$7 hours behind Tokyo, or alternatively, Tokyo is $7$7 hours ahead of Johannesburg.

From the table, Chile is $-4$4 GMT and Iran is $+3.5$+3.5. The difference is $+3.5--4=7.5$+3.54=7.5 hours. As $+3.5>-4$+3.5>4, this means that Iran is ahead of Chile. We can then say that Iran is $7.5$7.5 hours ahead of Chile, or alternatively, Chile is $7.5$7.5 hours behind Iran.

#### Practice questions

##### Question 1

In the following map, the colour of the region is used to indicate the time zone.

1. What is the time difference between Moscow and Dublin?

##### Question 2

Maria wants to know the time in Bangkok (UTC $+7$+7) when it is $11$11am in Athens (UTC $+2$+2).

1. Calculate the time difference between the two cities.

2. Hence find the time in Bangkok when it is $11$11am in Athens.

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##### Question 3

The following table shows the time of several locations relative to UTC.

UTC offset Location
$-5$5 New York, Cuba, Peru
$-4$4 Chile, Barbados, Brazil (west), Bolivia
$-3$3 Argentina, Brazil (east), Uruguay, Greenland
$-2$2 South Sandwich Islands
$-1$1 Azores
$0$0 UK, Ireland, Iceland, Portugal, Ghana, Liberia, Mali
$+1$+1 Algeria, Angola, Chad, Bosnia, Croatia
$+2$+2 Finland, Greece, Lebanon, Egypt, South Africa
$+3$+3 Russia (west), Saudi Arabia, Kenya, Madagascar, Iraq
$+3.5$+3.5 Iran
$+4$+4 Mauritius, United Arab Emirates, Armenia
$+4.5$+4.5 Afghanistan
$+5$+5 Pakistan, Maldives, Kazakhstan
$+5.5$+5.5 India, Sri Lanka
1. What is the time difference between Kenya and Mauritius?

2. When it is $1$1$7$7:$2$2$5$5 on the $15$15th day of the month in Kenya, what time is it in Mauritius? Give your answer in 24 hour time.

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## Applications of world time zones

There are many possibly applications, of which one main area is communications. For example, when making phone calls (or video calls) with people in other countries, whether it be for business, or calling family and friends, we need to ensure we are calling at appropriate times.

We can use the GMT of the two people involved, or even just the time difference, for scheduling calls. Another way is to use a website, like www.timeanddate.com, which provide various tools for scheduling your time.

One way of doing this is by using the "Current Time" of cities around the world. The table below is an excerpt of the list they might provide:

City Current Time
Madrid Fri $2:52$2:52 am
Managua Thu $6:52$6:52 pm
Manila Fri $8:52$8:52 am
Melbourne Fri $10:52$10:52 am
Mexico City Thu $7:52$7:52 pm
Miami Thu $8:52$8:52 pm
Minneapolis Thu $7:52$7:52 pm
Minsk Fri $3:52$3:52 am
Montevideo Thu $9:52$9:52 pm
Montreal Thu $8:52$8:52 pm
Moscow Fri $4:52$4:52 am
Mumbai Fri $6:22$6:22 am

From the table above we can see that the current time in Minsk is Fri $3:52$3:52 am, and the current time in Montevideo is Thu $9:52$9:52 pm. As it is currently Thursday in Montevideo, and it is currently Friday in Minsk, we can see that Minsk is ahead of Montevideo. More specifically, Minsk is $6$6 hours ahead of Montevideo.

#### Practice questions

##### Question 4

Valerie in Hobart has a phone conversation with Oprah in Manilla. Manilla is $2$2 hours behind Hobart time.

1. If Valerie calls at $10$10:$30$30am, what time is it for Oprah?

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2. If Oprah calls at $7$7:$30$30pm, what time is it for Valerie?

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##### Question 5

The rugby World Cup final is broadcast live across the world. One particular year it was held in Wales and the game started at $4$4 pm local time. Where Beth lives, it is $17$17 hours behind Wales. At what time did the game start in her local time?

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##### Question 6

The table shows the current time in major capital cities beginning with the letter $M$M.

Madrid Mon $2$2:$16$16 am Sun $6$6:$16$16 pm Mon $8$8:$16$16 am Mon $10$10:$16$16 am Sun $7$7:$16$16 pm Sun $8$8:$16$16 pm Sun $7$7:$16$16 pm Mon $3$3:$16$16 am Sun $9$9:$16$16 pm Sun $8$8:$16$16 pm Mon $4$4:$16$16 am Mon $5$5:$46$46 am
1. When it is $15$15:$10$10 Minsk time, what is the time in Madrid? Give your answer in $24$24 hour time.

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### Outcomes

#### ACMEM078

calculate time intervals, such as time between, time ahead, time behind