Time has a measurement system which was originally based on natural events such as the time it takes for the earth to circle the sun, approximately one year.
Units of Time

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

Feb

March

April

May

June

July

August

Sep

Oct

Novr

Dec

31

28*

31

30

31

30

31

31

30

31

30

31

*February has 29 days in Leap years, which as every 4 years (1992, 1996, 2000, 2004 ...)
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 (during the morning between midnight and midday) are followed by the letters am
 e.g. 3.42 am means 42 minutes after 3 o'clock in the morning.
 Times during the last 12 hours of the day (during the afternoon and evening between midday and midnight) are followed by the letters pm
 e.g. 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.
e.g. 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

Melbourne, Australia at about 2030 in winter 
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 6 hours and 15 minutes into seconds 
6 hours = 6 × 60 × 60 = 21 600 seconds 15 minutes = 15 × 60 = 900 seconds Total time = 22 500 seconds 
How many minutes are there between 7.35 am and 2.15pm the following day? 
From 7.35 am till 8.00 am = 25 minutes From 8.00 am till noon = 4 hours = 240 minutes From noon till 2. 00 pm = 2 hours = 120 minutes From 2.00 pm till 2.15 pm = 15 minutes Total time = 400 minutes 