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3.05 Time zones and the date line

Lesson

Time zones

The world is divided into many time zones that allow different places to set their own time. The time zones are based on geography but are mainly influenced by politics, so they can seem strange or misplaced. In order to keep track and compare the time in different places, each location is given a UTC offset (Coordinated Universal Time) which allows it to be compared to other places around the world. Greenwich, a city in England was chosen to be the timezone with UTC $+0$+0 because it lies at $0^\circ$0° longitude. For this reason UTC was previously called GMT (Greenwich Mean Time).

Time zones around the world with the UTC offset given below

In the image above the time zones are represented by coloured bars, with the UTC time given at the bottom of the bar. In general, countries to the east of the UTC $+0$+0 time zone are ahead in time compared to UTC $+0$+0 and will have a "$+$+" symbol before the number, for example, Sydney is in UTC$+10$+10 which is $10$10 hours ahead of UTC $+0$+0.

Countries to the west of the UTC $+0$+0 will have time zones that are behind and will have a "$-$", for example, Los Angeles is in UTC $-8$8, which is $8$8 hours behind UTC $+0$+0.

Some time zones are not a whole number of hours away from UTC. For example Adelaide uses UTC $+9:30$+9:30 (or UTC $+9.5$+9.5) representing the hours and minutes. These regions are striped in the image above.

We can also use tables that show the UTC time for countries.

City UTC
Vancouver $-8$8
New York $-5$5
Rome $+1$+1
Dubai $+4$+4
New Delhi $+5.5$+5.5 ($+5:30$+5:30)
Perth $+8$+8
Melbourne $+10$+10

The time zone can be read off the table for the corresponding city.

Worked examples

Example 1

From the image below, what is the UTC time zone Paris is in?

Think: We can look at the colour of the region the city is in, and find what UTC time it relates to.

Do: The city is in a yellow region, which corresponds to the nearest yellow band, which is highlighted below.

This corresponds to UTC $+1$+1. Paris will be $1$1 hour ahead of UTC $+0$+0.

 

Example 2

From the table provided what is the time in New York when it is $11:00$11:00pm in Dubai?

City UTC
Vancouver $-8$8
New York $-5$5
Rome $+1$+1
Dubai $+4$+4
Melbourne $+10$+10

Think: We can first find the time difference between the two cities and then use this to find whether New York is ahead or behind of Dubai and by how much.

Do: New York uses UTC $-5$5 and Dubai uses UTC $+4$+4. The difference in hours is $4-\left(-5\right)=9$4(5)=9 hours. From the offsets $-5$5 is smaller than $+4$+4 which means that New York is behind Dubai in time.

If the time is $11:00$11:00 pm in Dubai then it will be $9$9 hours earlier in New York, which is $2:00$2:00 pm.

Summary

The world is divided up into time zones which are labelled with a UTC offset. This specifies the number of hours behind or ahead that time zone is from UTC $+0$+0. We can use the numbers and sign of the UTC to find the time difference between two places.

Practice questions

Question 1

Consider the map and complete the following:

  1. When it is $17$17$:$:$00$00 in Cape Town what is the time in San Francisco?

    $19$19$:$:$00$00

    A

    $1$1$:$:$00$00

    B

    $7$7$:$:$00$00

    C

    $3$3$:$:$00$00

    D

    $9$9$:$:$00$00

    E

    $15$15$:$:$00$00

    F

    $19$19$:$:$00$00

    A

    $1$1$:$:$00$00

    B

    $7$7$:$:$00$00

    C

    $3$3$:$:$00$00

    D

    $9$9$:$:$00$00

    E

    $15$15$:$:$00$00

    F
Question 2

Consider the table of time zone information provided.

City UTC
Tijuana $-8$8
Mexico City $-6$6
Havana $-5$5
Kiev $+2$+2
Sri Lanka $+5.5$+5.5
Singapore $+8$+8
Seoul $+9$+9
  1. When it is $9$9$:$:$00$00 am in Mexico City what is the time in Sri Lanka?

    $2$2$:$:$30$30 am

    A

    $8$8$:$:$30$30 am

    B

    $8$8$:$:$30$30 pm

    C

    $3$3$:$:$00$00 am

    D

    $3$3$:$:$00$00 pm

    E

    $2$2$:$:$30$30 pm

    F

    $2$2$:$:$30$30 am

    A

    $8$8$:$:$30$30 am

    B

    $8$8$:$:$30$30 pm

    C

    $3$3$:$:$00$00 am

    D

    $3$3$:$:$00$00 pm

    E

    $2$2$:$:$30$30 pm

    F

 

The International Date Line

The International Date Line is the line that separates the two hemispheres on one side of the Earth at $180^\circ$180° longitude. It has an odd shape as it tries to avoid cutting through countries.

The International Date Line

Worked Example

EXAMPLE 3

What is the time and day in UTC $+12$+12 when it is $2:00$2:00 pm on Sunday in UTC $-11$11?

Think: We can calculate the time difference in the same way as normal. We take the difference of the two time zones.

The UTC $+12$+12 is ahead of UTC $-11$11, and there is a difference of $12-\left(-11\right)=23$12(11)=23 hours. is $23$23 hours ahead of. The time will be $1:00$1:00 pm, but on the Monday.

Reflect: When comparing time zones across the International Date Line, the change in time will be big enough that the day will normally change as well. Remember that it is always the countries with the positive UTC offset (like UTC $+12$+12) that will be ahead of the negative ones.

 

Time zones in Australia

Australia has several time zones. It is also a country that uses Daylight Saving Time in the summer months. From the first Sunday of October to the first Sunday of April, some of the Australian states and territories turn their clocks forward one hour. This happens at $2$2 am on the first Sunday of October and finishes on the first Sunday of April, turning an hour back at $2$2 am. Below is the map of Australia outside of DST and during DST. Notice that New South Wales (NSW), Victoria (VIC), South Australia (SA), the Australian Capital Territory (ACT) and Tasmania (TAS) all use DST time and the other states and territories do not.

Summary

Some Australian states and territories observe Daylight Saving Time (DST) in the summer months, turning their clocks forward one hour. The states that observe DST are New South Wales (NSW), Victoria (VIC), South Australia (SA), the Australian Capital Territory (ACT) and Tasmania (TAS).

Practice question

Question 3

When the time is $14$14$:$:$15$15 in Perth what is the time in:

  1. Sydney during No DST?

    $11$11$:$:$15$15

    A

    $12$12$:$:$15$15

    B

    $16$16$:$:$15$15

    C

    $17$17$:$:$15$15

    D

    $11$11$:$:$15$15

    A

    $12$12$:$:$15$15

    B

    $16$16$:$:$15$15

    C

    $17$17$:$:$15$15

    D
  2. Sydney during DST?

    $11$11$:$:$15$15

    A

    $12$12$:$:$15$15

    B

    $16$16$:$:$15$15

    C

    $17$17$:$:$15$15

    D

    $11$11$:$:$15$15

    A

    $12$12$:$:$15$15

    B

    $16$16$:$:$15$15

    C

    $17$17$:$:$15$15

    D

 

Outcomes

MA4-7NA

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