Articles of calendar computations

Weekend birthdays

My birthday this year (2011) is on a Friday. In most years, one’s birthday the following year is on the subsequent day of the week, and in that pattern, my birthday next year (2012) it is on a Saturday. However, due to 2012 being a leap year, my birthday in 2013 will be on Monday […]

Why is it that if I count years from 2011 to 2014 as intervals I get 3 years, but if I count each year separately I get 4 years?

I’m not a very smart man. I’m trying to count how many years I’ve been working at my new job. I started in May 2011. If I count the years separately, I get that I’ve worked 4 years – 2011 (year 1), 2012 (year 2), 2013 (year 3), 2014 (year 4). But if I count […]

How to find day of a date

As previously there were a method to find the date from the year 1893 to 2032 , which is very difficult to take a list of table always . Is there any other easy way to find the day of any date ?

Calculate which day of the week a date falls in using modular arithmetic

In Summer Wars the main character (he is a mathematician) calculates the day of the week of someone’s birthday (19/07/1992 is Sunday). I know (very) basic modular arithmetic but I can’t figure out how to do it. Can someone point me to the right direction? It seems fun to do.

birthday problem – expected number of collisions

There are many descriptions of the “birthday problem” on this site — the problem of finding the probability that in a group of $n$ people there will be any (= at least 2) sharing a birthday. I am wondering how to find instead the expected number of people sharing a birthday in a group of […]

Prove that every year has at least one Friday the 13th

Everyone knows Friday the 13th is regarded as a day of bad luck. Why does every year have at least one of this bad day?