6.04 International time zones and time differences

Time zones

The world has $25$25 standard time zones, 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.  

 

If you look at the coloured bar along the bottom of the map, you will see how the time zones should be broken up, according to the lines of longitude. If you compare this to the colours of the map, you will notice that the countries that follow that time zone aren't always perfectly within that zone. There are various reasons for this:

• some countries want to stay in synch 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 am in Paraguay. 

 

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^\circ$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	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 $+3.5--4=7.5$+3.5−−4=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

Question 2

Question 3

 

 

Applications of world time zones

There are many possibly applications, and 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

Question 5

Question 6

 

Calculating travel times between cities

When we're calculating how long a flight takes, we need to consider the local time in both cities. Similarly, if we want to know the local time in the departure/arrival city, we need to consider the time in one city and the flight time and then adjust the time based on time zones.

 

Practice questions

Question 7

Question 8

Question 9