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

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 Time is not usually measured in multiples of 10. 
Months
The number of days in a month varies for historical reasons.
Jan

Feb

March

April

May

June

July

Aug

Sept

Oct

Nov

Dec

31

28*

31

30

31

30

31

31

30

31

30

31

*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 −
Years
10 years

Decade

100 years

Century

1000 years

Millennium

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. 
24hour 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
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.
Examples

Answers

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. 
Safety

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

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. 
Clocks

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 −
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 −
Examples
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)