This tag is for questions about combinatorial and computational issues with regards to calendar systems. A typical, well-known mathematical result in this direction is John Conway's Doomsday Algorithm for computing the "day of the week" associated to a given date.
22
votes
5answers
1k views
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?
11
votes
2answers
234 views
Formula for occurrence of leap years in the Jewish calendar
Over at Judaism.SE, there was a discussion about a formula to determine leap years in the Jewish calendar. Basically, the calendar follows a 19-year cycle, and seven of those years -- 3, 6, 8, 11, 14, ...
11
votes
1answer
272 views
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 ...
11
votes
3answers
2k views
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 ...
5
votes
2answers
2k views
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 ...
3
votes
2answers
175 views
Five Fridays and Sundays on October
How to prove that if you take any 400 consecutive Octobers then exactly 14 % of those years have five Fridays and Sundays?
3
votes
1answer
65 views
Prime number based calendar: how to calculate intermediate values?
I'm trying to define a new [silly] calendar, because there aren't enough of them yet.
My calendar, so far, is specified:
epoch (orthodox) is the moment the leading edges of the dinosaur-killing ...
2
votes
1answer
63 views
In a certain year, January had exactly $4$ Mondays and $4$ Fridays. What was the day on $2^{\text{nd}}$ October the previous year?
In a certain year, the month of January had exactly $4$ Mondays and $4$ Fridays. What was the day on Gandhi Jayanti $(2^{\text{nd}}$ October $)$ the previous year?
I am not sure how to approach this ...
2
votes
2answers
105 views
Solve for numbers to appear on two six-sided dice
I have a small wooden calendar that uses two six-sided dice to display day of month. One die carries numbers 0, 1, 2, 6, 7, 8 and the other carries 0, 1, 2, 3, 4, 5. The six of course doubles as the ...
2
votes
3answers
75 views
Is there any fast way to get the number of a certain day in a week
I'm realy sorry, if this question is a bit stupid... But this is my first time on mathematics stackexchange.
Do you guys now for example, how to know the number of monday in a year?
2
votes
2answers
365 views
Jack's birthday riddle
Anytime each of three consecutive months has exactly four Fridays, Jack's birthday will fall in one of those three months. Which month is that?
2
votes
1answer
124 views
Periodicity of calendar
Let $A$ be the set of days in week, $A = \{Mon,Tue,Wed,Thu,Fri,Sat,Sun\}$, and $B = \{1,2,\dots,31\}$. For each date, we have a pair $(x,y)$, $x \in A$ and $y \in B$. Then we will have an infinite ...
2
votes
1answer
470 views
Hindu calendar (lunar) to Gregorian calendar
We have to convert the Hindu calendar (the lunar one) to the Gregorian calendar. According to the (Dutch) Wikipedia (sorry for that, but it has more information than other websites), it is based on he ...
1
vote
2answers
607 views
How often will the same date be on the same day of the week?
Is there any pattern in this (e.g. Once per five years)?
Is it a sliding window?
1
vote
3answers
1k views
How many odd days are there in 1600 years ? I calculated it but I guess the result is wrong?
I had to calculate the number of odd days in 1600 years. I have read the answer to be equal to 0. But I don't get it to equal to 0.
This is the way I am calculating the number of odd days in 1600 ...
1
vote
0answers
48 views
Calculating day on date [duplicate]
Possible Duplicate:
Calculate which day of the week a date falls in using modular arithmetic
Yesterday there was a guy on TV. He asked people in the audience at what date they were born and ...
0
votes
1answer
88 views
How to calculate Anniversary Billing for Uneven time frames?
What is the best way to calculate anniversary billing monthly with different lengths
We want to charge for storage say starting June 1st to July 1st and if the customer stays longer than July 1st we ...
-1
votes
1answer
247 views
Math Riddle Involving Birthdays [closed]
So someone asked me the following riddle:
If a girl is born in the 19th century, and on her first birthday she is 8. How is this possible? I think it's a simple mathematics problem but I can't find a ...
