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Questions tagged [calendar-computations]

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.

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What is the probability of someone being born both on a Good Friday that is also Friday the 13th

I was born on April the 13th 1990. The day was also Good Friday. I'm wanting to know what the probability/chances of being born on a Good Friday that is also a Black Friday is/are to have an ...
Dee Kay's user avatar
  • 51
1 vote
0 answers
39 views

What is the probability that the fourth business day of the month is Thursday? [duplicate]

This is question 3.16 from the book "Quant Job Interview Questions and Answers" by Mark Joshi et al. The solution given in the book states that for the fourth business day of the month to be ...
bobby202's user avatar
0 votes
0 answers
29 views

Calculating Calendar Adjustment

I have a data with the Date and Volume column like below, where I am trying to adjust the calendar, the dates are week-ending dates, hence some of the data is missed since May ended with a full week ...
noswear's user avatar
  • 109
0 votes
1 answer
109 views

Dates on which a day falls, in given month and year.

I was recently given the following problem on a test: Find the dates on which Sunday fell in July, $1776$. First, I noted the date on the day of test — $15$ October $2023$ to be a Sunday. Thereafter,...
Sahaj's user avatar
  • 2,554
1 vote
1 answer
59 views

Book that goes over mathematics of calendar computations?

What is a good book that goes over mathematics of calendar computations (e.g., calendar systems, date of Easter, etc.)?
Geremia's user avatar
  • 2,405
3 votes
0 answers
41 views

Friday the $13$ths with digits adding up to $13$

Three Friday the $13$ths with digits (in yyyy-mm-dd) adding up to $13$ include 2006-01-13, 2006-10-13, and 2011-05-13. So, how many such Friday the $13$ths (in the Gregorian calendar) are there from ...
Geoffrey Trang's user avatar
0 votes
0 answers
521 views

Is AD $4000$ a leap year or not?

There is a popular rule to determine which year is a leap year or not. The year is exactly divisible by four (with no reminder); If the year is divisible by $100$ (years ending in two zeros), it is ...
newview's user avatar
  • 109
-2 votes
1 answer
66 views

Is Date X + 365 days and Date X + 1 Year always will result in same date?

In particular, Date X + 12 Months Date X + 1 Year Date X + 365 days Will these result in all same dates at any point of X?
thqdqe's user avatar
  • 17
2 votes
3 answers
105 views

Is there a way to calculate the exact birthdate of a person based on a series of dates and ages? [closed]

I need to calculate the birthdate of an ancestor based on a list of dates and ages he had, the problem is that I stink at circular maths and rotating stuff. I have this dictionary in python: The left ...
Lerian Acosenossa's user avatar
1 vote
1 answer
87 views

Hash function from date to biweekly event

My trash gets picked up every other Tuesday. Now it's easy to remember what day of the week it is, but not so easy to remember if it's a skip week or not - especially if you miss it sometimes. You ...
evansrhonda's user avatar
2 votes
0 answers
38 views

Find the date at which a group of people will reach a target age sum

The goal is to find the date at which the sum of ages of a group of people will reach a certain number. Though I seem to have found an approximative solution, some issues remains, that I don't know ...
Elvex's user avatar
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0 votes
0 answers
73 views

How to converts total days passed since epoch from real Gregorian calendar to teorical Gregorian that all years are leap years?

Assuming: there is a "hypothetical Gregorian calendar" with same notations of the realistic (but no week days); therefore each date of the hypotetical calendar has the format "month ...
RHER WOLF's user avatar
  • 224
1 vote
0 answers
57 views

Calculate what day it will be 100 years [duplicate]

I'm trying to figure out how to calculate the day it will be in 100 years. I've written some python code that takes two inputs from the user, a year and day and then I'm trying to calculate what day ...
user2220115's user avatar
2 votes
1 answer
886 views

How exactly to calculate a Julian Date from today's date?

I have today discovered the intricacies of software dates. That led me to this page which shows: $$JD=int[365.25\; y]+int[30.6001\; (m+1)]+D+\frac{UT\_hours}{24.0}+1\,720\,981.5 \qquad$$ However, ...
Lance's user avatar
  • 3,694
1 vote
1 answer
66 views

Frequencies of gaps between years with $3$ Friday the 13ths

Years with $3$ Friday the 13ths in the Gregorian calendar usually occur with gaps of $3$ or $11$ years, except for two $6$-year gaps (between $2099$ and $2105$, and $2195$ and $2201$) and one $12$-...
Geoffrey Trang's user avatar
0 votes
1 answer
44 views

Odds $11$ to $3$ against a month selected randomly containing portions of $6$ different weeks

Here's a problem from my probability textbook: Show that the odds are eleven to three against a month selected at random containing portions of six different weeks. Here are some initial thoughts. ...
Emperor Concerto's user avatar
3 votes
2 answers
2k views

Reliable Formula for repetition of of calendar

I am trying to get a formula for repetition of calendars (for years to repeat with same day-date pattern) I could see the following rule in some of the online sites a) If given year is at 1st ...
Kiran's user avatar
  • 4,198
1 vote
1 answer
642 views

Conversion from epoch time to (dd/mm/yyyy) date format

The title pretty much explains my problem. I´m given the number of seconds that have passed since the 1st of January 1970 and I have to return today´s date in (dd/mm/yyyy) format. I know how to ...
Tomas Sexenian's user avatar
0 votes
1 answer
40 views

A formula to determine the number in a given sequence

I'm working with some specific numbers concerning a calendar calculation. Basically, 13 years in a sequence correspond to 13 different numbers: Year Given number Year 1 1 Year 2 10 Year 3 6 Year ...
Pablo Ivan's user avatar
1 vote
0 answers
27 views

Lunisolar calendar: what phase was the moon X years ago on the spring equinox?

How would I go around answering these questions (e.g., for $X=10,000$ -- IOW, historical observations are not available and the time is too long ago to rely on a fixed calendar): How many days ago ...
sds's user avatar
  • 4,533
1 vote
0 answers
25 views

Wikitext, #time parser function: Find last month with five Mondays

in Asphalt Wiki, we have templates for repeating events in the Asphalt games. These templates also display if an event is currently running. In one of the games, events are bound to days of the month ...
Guy Bukzi Montag's user avatar
1 vote
0 answers
121 views

Can an age or a difference between two dates be represented linearly?

I've found an interesting answer on StackOverflow when it comes to calculating someone's age based on that person's date of birth and the date at which the age is calculated. The calculation is ...
JansthcirlU's user avatar
1 vote
0 answers
227 views

Finding Anchor day of any year using Doomsday rule.

There is a formula or a method given by Sir John Conway to find any given year anchor day after year 1800 assuming that we are using Gregorian calendar .For example for finding anchor day (also known ...
Piyush choudhury's user avatar
0 votes
0 answers
56 views

formula for date as # day of the gregorian calendar_year (including leap years, excluding leap-seconds)

large to small could go 2021-Jan-02nd. Suppose want it in U.S. style, 01/02/'21, where 01/01 = January 1st = day#1 of the year (2021). 01|31 02|28,29 03|31 04|30 05|31 06|30 07|31 08|31 09|30 10|31 11|...
user avatar
9 votes
1 answer
3k views

Formula to find the day of any date in Gregorian calendar derivation.

My teacher taught an expression to find the day of any date in the Gregorian calendar. Expression: $$ \bbox[5px,border:2px solid red] { D\equiv {d+c_m+c_y+c+\left\lfloor\dfrac c4\right\rfloor \pmod7} }...
Wolgwang's user avatar
  • 1,563
0 votes
1 answer
938 views

Suppose today is Wednesday. What day of the week will it be $10^{{10}^{10}}$ days from now? [duplicate]

Question: Suppose today is Wednesday. What day of the week will it be $10^{{10}^{10}}$ days from now? Answer: This is what I have tried so far. I know that you need to find $10^{{10}^{10}}\text{mod} 7$...
Math Fanatic's user avatar
2 votes
1 answer
122 views

Computation of Easter date (UK statutory method)

The statutory method for the computation of Easter in the UK (and presumably also the USA) is contained in the Calendar (New Style) Act 1750. This is the same Act which moved the British Empire from ...
Steve's user avatar
  • 141
4 votes
2 answers
321 views

Question concerning the solution of a certain calendar problem involving modular arithmetic.

A question I was given is the following: A certain month on the calendar has 31 days, and it has an equal amount of Mondays and Wednesdays. How many different kind of days(Monday to Sunday) can be ...
Aiden Chow's user avatar
  • 2,845
2 votes
1 answer
625 views

How can I find day of a certain month when the sum of all thursdays is a certain number?.

The problem is as follows: The sum of all thursdays in certain month is $80$ and equal to the sum of all sundays in the following month, which has the same number of mondays than fridays it has ...
Chris Steinbeck Bell's user avatar
0 votes
2 answers
72 views

How can I find the name of the last day of a week in certain year when the name of a day of a month for that year is given?

The problem is as follows: Roger sees in his almanac that a certain month from a year the first day was monday and the last day was also monday. What day of the week, was the last day of that ...
Chris Steinbeck Bell's user avatar
0 votes
3 answers
50 views

How can I find the a certain birthdate of two people when one information given depends on the other?

The problem is as follows: Mike was born on $\textrm{October 1st, 2012,}$ and Jack on $\textrm{December 1st, 2013}$. Find the date when the triple the age of Jack is the double of Mike's age. The ...
Chris Steinbeck Bell's user avatar
0 votes
1 answer
687 views

Why the 28 year day-date repetition rule doesn't always follow?

I was studying the pattern of calendar and found out that the years repeat same day-date pattern over a period of 28 years but the pattern broke if we compare 1752 and 1780.(By day-date pattern i mean ...
Shashank Kumar Pandey's user avatar
0 votes
2 answers
2k views

What is the maximum number of years I have to wait till my birthday falls again on a Sunday?

Suppose my birthday fell on a Sunday. What is the maximum number of years I may have to wait till my birthday falls on a Sunday again? My intuition says 11 years: At the earliest, the next Sunday is ...
Flux's user avatar
  • 319
2 votes
2 answers
118 views

How many times in the future after the 1st of January of 2019 will a palindrome date happen?

I'm trying to solve the following problem: How many times in the future (before the year 10000) will after the 1st January of 2019 happen a palindrome date. Date is in the dx-my-ymxd forum, and ...
Peter F.'s user avatar
  • 463
-1 votes
2 answers
293 views

Must each new year always contain a Friday the 13th?

I am wondering if there is any way to determine whether it is possible to have a year that does not contain a Friday the 13th?
llewmills's user avatar
  • 117
0 votes
0 answers
50 views

When does adding 180 or 220 years give a date falling on the same day of the week?

Many dates in the Gregorian calendar repeat the day of the week after 180 and/or 220 years. In fact, this was already the case for October 15, 1582 (1582, 1762, and 1802 all had October 15 on a Friday)...
Geoffrey Trang's user avatar
2 votes
3 answers
3k views

How many years later the calendar of 2016 can be reused?

How many years later the calendar of 2016 can be reused? I was asked this question in an exam. I want to explain the term 'reuse' . If we ignore the holidays and our only purpose is to check the date ...
cmi's user avatar
  • 3,381
1 vote
1 answer
734 views

How to find the week day of (any) given date? [duplicate]

How to find the week day of any given date? Say we need to know in which week-day was June $25,2019$?
Hussain-Alqatari's user avatar
0 votes
1 answer
131 views

"New Year falls more often on Sundays than on Mondays"

This is a question from a hungarian math contest from the year 1948. It was Saturday on the 23rd October, 1948. Can one conclude that New Year falls more often on Sundays than on Mondays? Well, I ...
Vinyl_cape_jawa's user avatar
2 votes
1 answer
296 views

Number of occurrences of a generic recurring event between two dates

Intro Cron is a very common tool in computer science, which allows to schedule an event given a generic pattern written in a file called crontab. I'm researching on an algorithm to calculate the ...
Buendiadas's user avatar
5 votes
4 answers
2k views

Why don't we subtract out / ignore the leap year every 128 years? [closed]

When we add the extra day to the calendar every 4 years it's to "correct" the calendar under the assumption that the year's length is 365.25 days. Every year, you're .25 days off, so after 4 years, ...
Tyler T's user avatar
  • 59
0 votes
0 answers
64 views

I want to know what not integer numbers means in calendar?

I want to know what not integer numbers means in this case? if $30$ days $= 4$ weeks and $2$ days $4.2$ then why $20$ days does not correspond to $2$ weeks and $8$days $4,8$? being that: $\frac{30}{...
hunter 2050's user avatar
2 votes
5 answers
629 views

Prove That the Shortest Period of the Gregorian Calendar is 400 Years

I ask for help. I would like to resolve the following question: Let $A$ be the set of natural numbers greater than 1582. If, for every $Y\in A$ , the years $Y$ and $Y + n$ (of the actual calendar, ...
Paulo Argolo's user avatar
  • 4,240
0 votes
1 answer
2k views

Difference between two dates in math formula

Good evening guys, I have little problem, I have to calculate how many days do I have between two dates which contains specific day, month and even year, do you guys have any idea how should I do that?...
Sanady's user avatar
  • 11
2 votes
2 answers
72 views

How can I find the day of the week which an event happened after some time has elapsed?

The problem is as follows: An astronomer discovers a new comet while scouting the skies on Monday but decides to report his discovery $256$ days after. What day of the week did the astronomer ...
Chris Steinbeck Bell's user avatar
4 votes
1 answer
598 views

Math behind leap year cycle

Everybody knows that leap year is year which is divisible by 4. In case of century years, it has to be divisible by 400. The leap year cycle has 400 years (calendar repeat with the same day-date ...
IntelligentSandwich's user avatar
0 votes
1 answer
138 views

Tonalpohualli calculations

I was doing some math on the Tonalpohualli, the mexica ritual calendar. This calendar has 260 days divided in 13-days periods associated with 20 distinct symbols. Every 13-days period is called a ...
Pablo Ivan's user avatar
1 vote
1 answer
1k views

Formula to find the date given a specific year, month, and day of the week

I need a formula to get me to the first or third specified day of the week of the following year. So, here is the information I am presented. I have a year (y), a month (m) and a day of the week (n). ...
cain4355's user avatar
  • 113
2 votes
2 answers
209 views

Identical calendars

We can see, for example, that the years 2009 and 2015 have identical calendars. Similarly, 2000 and 2028. I read once that given any year X at most 28 years later there will be another year Y, with ...
Paulo Argolo's user avatar
  • 4,240
0 votes
0 answers
457 views

Calculating which day of the week a date falls on using Gauss's algorithm and modulo arithmetic

I've been told there's a straightforward way to determine which day of the week a date falls on, by first using Gauss's algorithm, to find the day of the week of 1st of January. Then this can be ...
Data's user avatar
  • 880