3.05 Time zones and the date line

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 time zone with UTC + \, 0 because it lies at 0\degree longitude. For this reason UTC was previously called GMT (Greenwich Mean Time).

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 time zone are ahead in time compared to UTC + \, 0 and will have a "+" symbol before the number, for example, Sydney is in UTC + \, 10 which is 10 hours ahead of UTC + \, 0.

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

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

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. We can use the numbers and sign of the UTC to find the time difference between two places.

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

For example, if it is it 2:00 pm on Sunday in UTC -11, then to find the time and day in UTC +12, would need to add 11+12=23 hours to get 1:00 pm on Monday.

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. It is always the countries with the positive UTC offset (like UTC +12) that will be ahead of the negative ones.

Examples

Example 1

Consider the map below:

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

Worked Solution

Create a strategy

Find the time difference between the two cities.

Apply the idea

Based on the map, Cape Town is in UTC +2 so it is 2 hours ahead UTC +0. While San Francisco is in UTC -8 so it is 8 hours behind UTC +0. So San Francisco is 10 hours behind Cape Town. So we need to subtract 10 hours from the given time.

\displaystyle \text{San Francisco time}	\displaystyle =	\displaystyle 17:00 - 10 \text{ hours}	Subtract the 10 hours
	\displaystyle =	\displaystyle 7:00	Evaluate

It will be 07:00 in San Francisco.

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Example 2

Consider the table of time zone information provided.

City	UTC
\text{Tijuana}	-8
\text{Mexico City}	-6
\text{Havana}	-5
\text{Kiev}	+2
\text{Sri Lanka}	+5.5
\text{Singapore}	+8
\text{Seoul}	+9

When it is 9:00 am in Mexico City what is the time in Sri Lanka?

Worked Solution

Create a strategy

Find the time difference between the two cities

Apply the idea

Mexico City is in UTC -6 so it is 6 hours behind UTC +0. While Sri Lanka is in UTC +5.5 which means it is 5.5 hours ahead UTC +0. So Sri Lanka is 6+5.5=11.5 hours or 11 hours and 30 minutes ahead of Mexico City. We need to add 11.5 hours to the time in Mexico City.

\displaystyle \text{Sri Lanka time}	\displaystyle =	\displaystyle 9:00\text{ am} + 11.5 \text{ hours}	Add the 11.5 hours
	\displaystyle =	\displaystyle 8:30 \text{ pm}	Evaluate

When it is 9:00 am in Mexico City, it is 8:30 pm in Sri Lanka.

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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 2:00 am on the first Sunday of October and finishes on the first Sunday of April, turning an hour back at 2:00 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.

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).

Examples

Example 3

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

a

Sydney during No DST?

Worked Solution

Create a strategy

Refer to the left side of the image above.

Apply the idea

Sydney (in NSW) is UTC +10 while Perth (in WA) is UTC +8. This means that Sydney is 2 hours ahead of Perth.

\displaystyle \text{Sydney time}	\displaystyle =	\displaystyle 14:15 + 2 \text{ hours}	Add the 2 hours
	\displaystyle =	\displaystyle 16:15	Evaluate

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b

Sydney during DST?

Worked Solution

Create a strategy

Refer to the right side of the image above.

Apply the idea

Sydney (in NSW) is now in EDT which is UTC +11 while Perth (in WA) is still in UTC +8. This means that Sydney is 3 hours ahead of Perth.

\displaystyle \text{Sydney time}	\displaystyle =	\displaystyle 14:15 + 3 \text{ hours}	Add the 3 hours
	\displaystyle =	\displaystyle 17:15	Evaluate

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