This article was initially published on International Women’s Day in 2026. That was 8 of March, 2026.
Or was it 19 of Adar I, 5786? Maybe 23 of February, 2026? Perhaps 19 of Ramadan, 1447?
Actually, all of the above!
And also 2026-W10-7.
In the beginning…
It all started a few months ago, when someone at the synagogue asked what day is Rosh Hashanah, which is the first day of the new year on the Hebrew calendar. Someone answered, “same as always: 1 of Tishrei.” This reminded of something I had read years ago on Calendrical Calculations by Nachum Dershowitz and Edward M. Reingold. The authors had encountered a similar situation years earlier, when someone asked what day begins Hanukkah (the Jewish festival of lights) on a given year, someone answered, “same as always: 25 of Kislev,” and so they began a study on the different calendars in use.
All my life I’ve been celebrating Jewish holidays in different days on the civil calendar. Same with my Muslim friends, who celebrate (among other important events) Ramadan (beginning 18 of February, 2026) and Eid al-Fitr (19 of March, 2026) in different days, and my Indian friends, with their holidays and festivals as well. Recently, I realized my Orthodox friends were not celebrating Christmas on 25 of December, but 7 of January of the following year.
In this article, I’m exploring the different calendars actually being used nowadays. In parallel, I’m building an Elixir library (source on Codeberg) with the calculations to convert from one to another.
A multitude of calendars
Most of which are still in use.
Julian calendar
The Julian calendar was proposed by Julius Caesar himself (hence the name) and took effect on 1 of January, 45 b.C.E., having prevailed for more than 1,600 years in the Roman Empire and most of the Western world. Because each year is approximately 365¼ days long, there are 3 years with 365 days and one with 366, for which an extra day in February is added.
But there is a problem: the year is approximately 365¼ days long, but not precisely. Actually, it’s a bit shorter than that. Science doesn’t care for our preconceptions and continues to do its thing. With time, people began to realize the seasons were changing when they were not expected, just like the lengths of days and nights. That’s because the Julian calendar gains one day every 128 years.
Gregorian calendar
To address this discrepancy, Pope Gregory XIII promulgated a new calendar, which is adopted as the civil calendar by most countries. The main motivation was that the March equinox was occurring much earlier than March 21, which is necessary for calculating the date of Easter. To correct it, Thursday, 4 of October, 1582 was followed by Friday, 15 of October, 1582.
The Gregorian calendar follows the same principles as the Julian calendar, with a difference: to be considered a leap year, not only the year must be divisible by 4, but if also by 100, then it must also be by 400. Yeah, that’s confusing, so let’s see a few examples: 1600 was a leap year; 1700, 1800, and 1900 were not; 2000 was; 2100 will not. Please remind your grandchildren to inform their children.
From 1901 through 2099, the Julian calendar is 13 days behind the Gregorian calendar.
ISO calendar
The ISO week date system is part of the ISO 8601 date and time standard issued by the International Organization for Standardization. It indicates the year in the Gregorian calendar where Thursday falls, and also indicates the week and day numbers. Each year has 52 whole weeks, adding a 53rd whole week for when the Gregorian year contains 53 Thursdays. Why this obsession with Thursdays?
That’s because Thursday is right in the middle of the week, so the year that contains a Thursday also contains the majority of the week. It also means the first week of the year is the one that contains 4 of January on the Gregorian calendar. As you can expect, ISO and Gregorian years may not be the same for dates close to 1 of January.
The first day of the week is Monday (day 1) and the last one is Sunday (day 7).
Dates are represented with 4 digits for the year, followed by a W and 2 digits for the week, and the day at the end. Optionally there can be dashes to the left of the W and of the day to make it more readable.
In the end…
Some people say, “it’s 5 o’clock somewhere” as an excuse to drink wine any time. I’m sure there exist more calendars than I’m accounting for, which means it must be my birthday somewhere. I’ll have some cake.
Revision history
- 2026-03-08: Original posting date.
- 2026-03-09: Julian and Gregorian calendars.
- 2026-03-11: A few corrections.
- 2026-03-12: ISO calendar.
Note: all dates are on the Gregorian calendar.