# Timelines

Lesson

## How many days in a month?

We normally take an average and say that a month has 30 days but not all months are the same.

Month Number of Days
January 31
February 28 (29 in a leap year)
March 31
April 30
May 31
June 30
July 31
August 31
September 30
October 31
November 30
December 31

Some people use this adage to help them remember:

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

Or you can use the knuckle method, where the months on the knuckles have 31 days and the months in the grooves have 30 (except February).

#### Example

What is the date 45 days after 29 December?

1. 13 February

A

11 February

B

14 February

C

12 February

D

13 February

A

11 February

B

14 February

C

12 February

D

## Using a Timeline

History is a timeline. Australia, like most other countries follows the Gregorian calendar, which is based on the Christian calendar. If you think of the year "0," time is divided before and after this. Anything after this point (just like when we count up using integers) is in the Anno Domini (AD) Era, which is also called the Common Era (CE). Anything before this "0" year (similar to negative numbers) is in the Before Christ (BC) Era or the Before Common Era (BCE).

#### Example

What is the year $223$223 years before $82$82 AD (CE)?

## What millennium are we in?

A millennium is $1000$1000 years. Although there is some debate about whether we should start measuring millennia from year "0" or year "1," it is commonly accepted to start from year "1." So the first millennium went from 1 AD (CE) to 1000 AD (CE) and the second millennium went from 1001 AD (CE) to 2000 AD (CE). We are currently in the third millennium.

#### Example

What millennium is 6426 in?

1. The $\editable{}$th millennium

## What century are we in?

A century is $100$100 years. We can work out what century a year is in using the same method as when we are working out what millennium a year is in. 1 AD (CE) to 100 AD (CE) is the first century, 101 AD (CE) to 200 AD (CE) is the second century and so on. We are currently in the 21st century.

#### Example

What century is 2518 in?

1. The $\editable{}$th century

## Leap years

A year is actually about $365.25$365.25 days so to make up for these quarter days that we miss, a leap year occurs every $4$4 years. A leap year has an extra day, making $366$366 days instead of the usual $365$365 days.

So...how can you tell what year is going to be a leap year?

3 STEPS

1. Check to see if the year is evenly divisible by $4$4.  If it is then it is a leap year - except if....
2. The year is also evenly divisible by $100$100 then it isn't - except if....
3. The year is also evenly divisible by $400$400, then it is.

So the year $2000$2000.  1) is divisible by $4$4 so it looks like a leap year but as per 2) it is divisible by $100$100 so maybe it isn't - but as per 3) because it is divisible by $400$400 then it is.

The year $2100$2100.  1) is divisible by $4$4 so it looks like a leap year, but as per 2) it is divisible by $100$100 so maybe it isn't, so as per 3) we check if it is divisible by $400$400 - it isn't so $2100$2100 is not a leap year.

The year $1988$1988.  1) is divisible by $4$4 so it looks like a leap year.  2) As we can immediately see that it isn't divisible by $100$100, we don't need to look any further.  It is definitely a leap year.

The year $2017$2017.  1) is not divisible by 4 - so it isn't a leap year at all.