# Measurement: Time

## The SI unit for measuring Time is the second, s.

### A second was originally calculated as 1/86400 th of a day, but this was not very accurate as the rotation of the Earth can be irregular. Nowadays, the second is derived from energy levels of an atom. In the UK an Atomic Clock at the National Physics Laboratory is accurate to one second in 138 million years! Go to BBC News - Science and Environment to find out more.

For the 11 Plus, your child needs to know how to read the time and to be able to solve problems involving time.

We measure time in seconds, minutes, hours, days, weeks, months and years. Your child needs to remember that there are:

### 365 days in 1 year (366 days in a leap year)

A 'Leap Year' occurs every 4 years because the actual time that the Earth takes to travel around the Sun is 365¼ days - so after every 4 years we have an extra day, the 29th February, which is added to the year to ensure that the seasons are synchronized.

If a year is divisible by 4 it is a leap year. (except for some Century years)

The next leap years are 2016, 2020 and 2024 which also coincide with the years that
the Summer Olympics take place.

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## 30 days has September, April, June and November. All the rest have 31, Except February alone, Which has but 28 days clear And 29 in each leap year.

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## Analogue Time - am/pm - The 12 Hour Clock

Analogue Time splits the day into two halves of 12 hours.

From midnight to midday - am - (Ante Meridiem - which is Latin for 'before noon'.)
From midday to midnight - pm (Post Meridiem - which is Latin for 'after noon'.)

This overcomes the problem that there are 2 times during the day when the hour is the same, which is why we must add 'am' or 'pm' to the time.

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## Digital Time - The 24 Hour Clock

Digital time uses the full 24 hours of a day - so there is no problem of having the same hour twice in a day (there is no 'am' or 'pm'). The day starts at midnight, 00:00 and counts
through in hours and minutes until one second before midnight, 23:59.
The 24-hour clock always uses 4 digits.

7:35 am = 07:35 .... 9:21 am = 09:21 .... Midday = 12:00 .... 1:20 pm = 13:20 .... 7:35 pm = 19:35
(don't forget to include the zero for 07:35 and 09:21 etc)

To convert from Analogue to 24-hour time - for any time after 12:59 pm just add 12.
(3.00pm becomes 3 + 12 = 15:00 ..... 5:30pm becomes 5 + 12 = 17:30)

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## 'Past' and 'To' the hour

When using Analogue time, any minute up to 30 is said to be 'past' the hour that has already gone ... 09:26 would be '26 minutes past nine'. Any minute after 30 minutes is said to be 'to' the next hour ... 09:36 would be '24 minutes to 10'.

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## Halves and Quarters

If the minute hand is on 15 it is usual to say quarter past the previous hour.
If the minute hand is on 45 it is usual to say quarter to the next hour.
If the minute hand is on 30 it is usual to half past the previous hour.

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Practice with this 12-Hour clock animation

Also from 'Maths is Fun'

From 'Mathsblog'