Miscellaneous calendar converters

I like making silly calendar converters. It's fun! Here's a list of all the ones I've made and put online, ranked roughly from most to least useful. Links marked with a star are based on "Calendrical Calculations: The Ultimate Edition" by Edward M. Reingold and Nachum Dershowitz and the JavaScript library "calendrica-js" by Sarabveer Singh based on the aforementioned book.

• Islamic calendar
Perhaps the first useful calendar converter I have ever made. Note that this is the easier to predict tabular version, not the more traditional look-at-the-sky version, so dates may be off by a day or two.
• Hebrew calendar
Officially the second useful calendar converter I have ever made (thanks, Gauß!). As it happens, it's the first day of Passover 2024 when I write this so... yay, I guess.
• Chinese calendar ⭐
My original plans to make this converter failed when I realised the intricacies involved in calculating it (astronomically accurate solstices and moon phases? Yeowch!). Once again, my day was saved by university library access.
• Bahá'í calendar ⭐
Made during Ridvan 2024 as a proof of concept to test the calendrica-js library. This version is proleptic, as in it applies the 2015 calendar system to dates prior to 2015. Personally, I don't mind that.
• Mayan calendar
A classic, though not very interesting computation-wise.
• ISO week date calendar
Used in bureaucracy, business and even food packaging sometimes. Every year has either 52 or 53 weeks, and every year starts on the Monday closest to 1 January. There are no months. A very similar calendar, which assigns whole weeks to individual months instead of years in much the same fashion, and is therefore much less useful and not officially recognised anywhere, can be found here.
• French republican calendar ⭐
Vive la révolution and some such, I don't know.
• Babylonian calendar
A modern extrapolation. Years are counted from the reign of Nabonassar. Months are calculated via an Easter date algorithm (see below), and no attempt has been made to closely replicate any error and irregular intercalation known from historical records. Inspired by and made to loosely be in sync with @BabylonDate by Neil Freeman.
• Dozenal solstice calendar ⭐
A calendar I found via random internet scrolling, by Paul Rapoport. Meant to be used together with a base-12 number system, but not awful even without it. Years are counted from 9564 BC, supposely the most recent year in which the perihelion and Northern summer solstice fell on the same day, though that seems to be incorrect. Dates of solstices are determined via calendrica-js.
• Martian calendar
The Martian calendar that I use within the Big Timeline. Not the Darian calendar, but instead based on the Martian Business Calendar by Bruce Mills. Years are counted from 1955, as established in Clancy (2000) and subsequently adopted as consensus in the field. (Why don't more Martian calendars do this?!)
• Easter calendar
Did you know that the Gregorian calendar has a (very janky) way of keeping track of the moon? This jank is used to determine when Easter Sunday is each year. You can also use it as a lunisolar calendar of its own right. I don't know why you would, but you definitely can. Changes made by me: Months are named after the Hebrew calendar, with "Nisan" refering to the month that Easter occurs in. Some tiny amounts of jank regarding missing or superfluous new moons around New Year have been removed based on Roegel (2014).
• Saudi Solar Hijri calendar
Ostensibly used in Saudi Arabia for fiscal purposes from 1987 onwards, at least if this page from Utrecht University is to be believed. I couldn't find any other source for its existence, and it presumably fell out of use after the Gregorian calendar was adopted in 2016. Effectively a reskin of the Gregorian calendar, though it's similar to the Iranian Solar Hijri calendar in a bunch of ways.
• 1982 Talossan calendar
A reskin of the Gregorian calendar, briefly adopted in the micronation of Talossa in 1982 and abandoned soon after since even the creator himself was confused by it.
• Tonal calendar
Another reskin of the Gregorian calendar, proposed by John W. Nystrom in 1862. The main gimmick is that it's base-16, or "tonal" as Nystrom called it. He also proposed his own hex digits that I have not bothered reproducing; this converter uses the conventional digits from 0 to F. Nystrom refered to the beginning of his calendar as "Christmas", even though it's... not. I don't know what's up with that.
• Positivist calendar
Yet another reskin of the Gregorian calendar, this time by August Comte. A year has 13 months, each with 4 weeks of 7 days, plus an extra holiday commemorating the dead (and holy women on leap years) at the end that don't belong to any week. Fun fact: Asgardia uses a very similar calendar to this one.
• Boryaworld calendars
A whole bunch of fictional calendars used for a group worldbuilding project. Hosted on a friend of mine's website.