Time is measured using units which depend on two events to do with the sun and the earth.

  • The time it takes the earth to go around the sun once is called a year.
  • The time it takes for the earth to spin on its axis for one turn is called a day.

Unfortunately, the number of days taken for the earth to move around the sun is about 365.25, not a whole number, so that is why every 4 years we add one day onto the year (29 February) and this makes up for the four extra quarters. These special years are called leap years. The year 2000 was a leap year.

The year 2000, means 2000 AD, where AD is Latin standing for Anno Domini and means 2000 years after the birth of Christ. Any years before the birth of Christ are labelled BC. e.g. 136 BC means 136 years before the birth of Christ.

Units of Time

The basic unit for time in the metric system is the second.

1 minute = 60 seconds
1 hour = 60 minutes
1 day = 24 hours
1 week = 7 days
1 year = 12 months = 365.25 days

Time is not usually measured in multiples of 10.



The number of days in a month varies for historical reasons.


*February has 29 days in Leap years, which are every 4 years (1992, 1996, 2000, 2004 ...)

To help you remember the lengths of these months − Time_03.gif


10 years
100 years
1000 years


Daily Time

There are two systems for telling the time.

am and pm

Times during the first 12 hours of the day are followed by the letters am

(during the morning between midnight and midday)

3.42 am means 42 minutes after 3 o'clock in the morning.

Times during the last 12 hours of the day are followed by the letters pm

(during the afternoon and evening between midday and midnight)

5.26 pm means 26 minutes after 5 o'clock in the afternoon.

24-hour clock

The hours of the day are numbered from 0 to 24, starting with 0 as the first hour after midnight.


0047 means 47 minutes past midnight

0800 means 8 o'clock in the morning

1530 means half past three in the afternoon

2307 means 7 minutes after 11 o'clock at night



It is midday or noon.

Special times

Midday is written 1200 or 12.00 pm or noon or midday and 
Midnight is written 0000 or 12.00 am or midnight.

Calculations involving Time Runners.jpg

Typical calculations involving time include finding out how many minutes there are between two times or how many seconds there are in a number of days.

Change 3 hours and 20 minutes into seconds

3 hours = 3 x 60 x 60 = 10 800 seconds

20 minutes = 20 x 60 = 1 200 seconds

Total time = 12 000 seconds

How many minutes are there between 1000 and 1530 on the same day?

From 1000 to 1500 = 5 hours = 300 minutes

From 1500 to 1530 = 30 minutes

Total time = 330 minutes

How many minutes are there between 8.30 pm and 3.15 am on the following day?

From 8.30 pm till 9.00 pm = 30 minutes

From 9.00 pm till midnight = 3 hours = 180 minutes

From midnight till 3. 00 am = 3 hours = 180 minutes

From 3.00 am till 3.15 am = 15 minutes

Total time = 405 minutes


Daylight Saving Time

In many countries around the world, in spring, standard time is changed by moving the clocks "forward" by one hour. At the end of the summer the clocks are "put back" by one hour to standard time. The main purpose of Daylight Saving Time (called "Summer Time" in many places in the world) is to make better use of daylight.

Advantages of Daylight Saving
Extra Daylight
Daylight Saving means that instead of the day getting light at say 6.00 am it will now get light at 7.00 am. This means an extra hour of daylight in the evening for leisure activities.
Saves Energy
Energy is saved especially in the evening because less electricity is used for lighting and appliances. In New Zealand, power companies have found that power usage decreases 3.5% when daylight saving starts.
Several overseas studies have found that daylight, almost certainly because of improved visibility, decreases, by about four times, the likelihood of pedestrians being killed on the roads.
Disadvantages of Daylight Saving
Farmers often do not like daylight saving because they have to work longer in the dark in the mornings and it disrupts the routines of themselves and their animals.
Many people complain because they have to keep changing the times on all of their clocks and watches!

This website gives a full history of daylight saving and the dates of daylight saving around the world.


Time Zones

Because the earth is rotating, the sun rises at different times as you move around the earth. This means that in order for daylight to begin in the morning and end in the evening, the time has to be adjusted. These adjustments are done using time zones.

For example, when it is 0700 in Auckland and the sun is rising, it is still dark in Sydney and the sun is still two hours away from rising SO ... Sydney is in a time zone which is two hours behind Auckland.

i.e. When it is 0700 in Auckland it is 0500 in Sydney.

As you move around the earth in a WESTERLY direction the sun will be rising later so these countries will be further behind Auckland and Sydney time. This table gives some examples (these times do not allow for daylight saving).

Perth Western Australia Auckland time minus 4 hours
New Delhi India Auckland time minus 6.5 hours
Rome Italy Auckland time minus 11 hours
London England Auckland time minus 12 hours
New York USA Auckland time minus 17 hours
Los Angeles USA Auckland time minus 20 hours
Hawaii USA Auckland time minus 22 hours

A world clock, showing the times in all of the major cities − Time_05.gif

A line of longitude just to the east of New Zealand is called the International Date Line and if you cross this line on an airplane or ship the date changes.

i.e. If you are travelling from New Zealand to America − the date goes back one day.
If you are travelling from America to New Zealand − the date goes forward one day.

For a map of the different time zones around the world − Time_05.gif

When it is 1300 in Auckland, it is 0200 in Rome. (1300 − 11 hours)
When it is 1600 in Auckland it is 0400 in London (1600 − 12 hours)
When it is 1100 in Auckland it is 1300 in Hawaii on the previous day! (1100 − 22 hours)