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Time

60 seconds = 1 minute
60 minutes = 1 hour
24 hours = 1 day
7 days = 1 week
52 weeks = 1 year

365 days = normal year
366 days = leap year

Leap years are divisible by 4
Centuries are not leap years unless divisible by 400

Convert units of time NRich - History of time measurement

Gregorian Calendar

Introduced by Pope Gregory XIII in October 1582, this replaced the Julian calendar
Adopted in Great Britain in September 1752 - 2nd September 1752 was immediately followed by 14th September 1752.

January   31 days
February   28 days (29 in leap year)
March     31 days
April        30 days
May     31 days
June          30 days
July         31 days
August     31 days
September 30 days
October   31 days
November 30 days
December 31 days

12 hour and 24 hour clock

12 hour clock times are written with
am to represent morning
and pm to represent afternoon.
A colon is used to separate minutes.

Examples

9 :30 am is half past nine in the morning.
9: 37 pm is 23 minutes to ten in the evening.

24 hour clock times are written with 2 digits to show hours
and a colon to separate minutes.

Examples

09 :30 is half past nine in the morning.
21: 37 is 23 minutes to ten in the evening.

Military usage of 24 hour clock times
drops the colon
and adds the word hrs.

Examples

09 30 hrs is half past nine in the morning.
21 37 hrs is 23 minutes to ten in the evening.

Converting between 12 hour and 24 hour clock times

face

To convert from 12 hour clock times to 24 hour clock times,

drop the am or pm
add 12 to the hours if the time is in the afternoon ( pm)

Examples

9:30 am = 09:30 or 09 30 hrs
3.15 pm = 15:15 or 15 15 hrs

To convert from 24 hour clock times to 12 hour clock times,

subtract 12 from the hours if the 24 hour time is between 13:00 and 24:00 or 13 00 hrs and 24 00 hrs
drop the hrs if it was there
add am if the 24 hour time is is between 00:00 and 11:59
add pm if the 24 hour time is is between 12:01 and 24:00

Examples

03:15 = 3:15 am
23:23 = 11:23 pm
11 27 hrs = 11:27 am
21 00 hrs = 9 pm