Month Numbers for the 2010s Part 1 - 2010 - 2014

Following on from this previous post, here's a shortcut to help you to work out what day any particular date falls on in any year between January 1 2010 and December 31 2019: the 2010s. I will do something for the 2020s later; this is to help you to work out the days that dates in the Teens fall on.

This is the guide to the first five years of the Teens, between 2010 01 01 and 2014 12 31. The second part covers the five-year period between 2015 01 01 and 2019 12 31.

The Method

The method, as explained in the older article, is simple. First, reduce the date of the month to a number below seven by removing the highest multiple of 7 from it. Thus the 23rd of a month yields the date number (23 - 21) = 2.

Second, add to this the month number, followed by the year number, as determined in the tables in the previous post. The month numbers for all years are the same:-

Month	Month Number
January	0
February	3
March	3
April	6
May	1
June	4
July	6
August	2
September	5
October	0
November	3
December	5

So, for instance, for the 23rd of September, add the month number for September (5) to the reduced date number (2); that yields 7.

Now adding the year number for the given year results in a number which can be reduced by further multiples of 7. Let's take the example of 23rd September 2014, chosen because 2014's year number is 2.

The date number + month number are 7; 7 + 2 = 9. Reducing this number yields (9 - 7) = 2.

Lastly, this final figure yields the day of the week on this lookup table.

Day Number	Day of The Week
0	Sunday
1	Monday
2	Tuesday
3	Wednesday
4	Thursday
5	Friday
6	Saturday

In the above example, 23rd September 2014 falls on a Tuesday.

The year number for each year in this decade are worked out by adding the difference between the year and the last leap year, which in this case is 2008 (before the scope of this article) and 2012. The leap year in the second article falls in 2016.

The year numbers for 2010 to 2014 are:-

Year	Year Number
2010	4
2011	5
2012	0
2013	1
2014	2

2012, being a leap year, has its own special rule: when working out a date in January and February in a leap year, after adding the month number subtract 1 before reducing the number by casting out the 7s.

Let's take 17th February 2012 (leap year: year number 0) as an example. 17 - 14 = 3; 3 + 3 = 6; 6 - 1 (for the leap year) = 5; 5 + 0 = 5. 17th February 2012 falls on a Friday.

Let's take a shortcut, now, and put the month numbers and year numbers together, preloading them before using them to work out the days any date falls on. These numbers have been reduced, where the adjusted numbers come to 7, 8 or 9 (a number reduced to -1 is changed to 6, etc).

Month	Unadjusted	2010	2011	2012	2013	2014
January	0	4	5	6	1	2
February	3	0	1	2	4	5
March	3	0	1	3	4	5
April	6	3	4	6	0	1
May	1	5	6	1	2	3
June	4	1	2	4	5	6
July	6	3	4	6	0	1
August	2	6	0	2	3	4
September	5	2	3	5	6	0
October	0	4	5	0	1	2
November	3	0	1	3	4	5
December	5	2	3	5	6	0

Don't worry! You won't have to memorise the whole table above! Just the month and year tables. The big table just contains the month and year figures already worked out for you.

Some examples using the big table:-

June 30th, 2013

30 - 28 = 2. The figure for June 2013 is 5. 2 + 5 = 7; 7 - 7 = 0. June 30 2013 fell on Sunday.

April 26th, 2012

26 - 21 = 5. The figure for April 2012 is 6. 5 + 6 = 11; 11 - 7 = 4. April 26 2012 fell on Thursday.

August 8th, 2014

8 - 7 = 1. The figure for August 2014 is 4. 4 + 1 = 5. August 8 2014 will fall on a Friday.

Now try it out for yourselves. Work out your birthdays in 2011, 2013 and 2014. Now work out your friends' or relatives' or loved ones' birthdays in 2010 and 2012. Make a game of it.

Practice on the following dates:-

July 4 2014	January 21 2012	September 16 2010
February 13 2012	March 13 2012	August 27 2011
October 3 2013	May 4 2010	December 4 2010
June 19 2013	April 26 2010	November 1 2010

Part 2 tomorrow, covering the years 2015 to 2019.