The **1911 Revolution** *(October 10 1911 to February 12, 1912)* signalled the end of the Qing Dynasty in China, the fall of the last emporer and both Taiwan and China’s beginnings as republics. Taiwan has treated this as its Independence Day, which it celebrates on October 10 *(“Ten Ten Day”)*. The communists in China celebrate October 1 *(the day the CCP took over in 1949)* and avoid the 10th.

I was here ten years ago for Taiwan’s hundredth anniversary *(also a Sunday)*, and can say I wish I had been on vacation in the Philippines instead. The amount of noise from fireworks that weekend was beyond deafening. My headaches lasted for days.

As you might have noticed, 2021 is 110 years in Taiwan’s domestic calendar. This means that on Friday in 2022, about an hour before noon, the calendar and clocks will read:

**111, 11/11, 11:11:11**

My students had a laugh with that.

Numberphile also did a video recently on calendar calculation, vis-a-vis telling the day of the week by the date alone. James Grime gives a system developed by the late mathematician John Conway, though others could be developed.

Conway used what he called “doomsday numbers” to determine the day of the week without having to calculate or remember many weeks or months. I checked, and it’s true.

**In every calendar year, the following dates are ALWAYS the same day of the week**, regardless of the year. For all my obsession with calendars and calendar reform, I had never noticed this before.

**All even double digit dates after February (4/4, 6/6, 8/8, 10/10, 12/12) **

**“Working 9 to 5 at 7/11” (5/9, 9/5, 7/11, 11/7)**

**January 4th in a leap year, or 3rd in most years
**

So no matter what date of the year it is, you’re only a few weeks away from a date that a person gives you. The video also mentions **Pi Day (March 14)** as being one of the above dates. Not mentioned in the video below is the **last day of February (28 or 29)**, regardless of regular or leap year.

There are only 28 possible calendars, and calendars run on 28 year cycles. If you can remember the first day when these cycles begin *(1900 which is not a leap year; 1600, 1628, 1656, 1684, 1814, 1842, 1870, 1898, 1916, 1944, 1972, 2000, 2028, etc. all start on a Saturday)*, it’s not hard to calculate the first day of the year. *[Correction: 1898 starts on a Saturday, not 1900, 18 years until 1916. The 17th and 18th centuries start to get tricky, every 28 years from 1695 to 1791.]*

So let’s say *(for example)* you want to know **the day of the week for Hallowe’en in 1980**. 1972 started on a Saturday, so eight years plus two leap years makes 10 days (10 mod 7 = 3), thus 1980 started on a Tuesday. January 1 to March 31 is 91 days in a leap year (91 mod 7 = 0), so April 1 is also a Tuesday, and by extension April 4 (4/4) is a Friday and so is 10/10. Hallowe’en (10/31) is exactly three weeks after 10/10, thus also a Friday.

But bleeping hell, I hate their ableist video editing.

WMDKitty -- Survivor says

Hold up. How is 1900 not a leap year? I am confused.

lochaber says

Um… I hope this isn’t a dumb question, as I’m not terribly good at noticing this stuff, but would anyone mind explaining the ableist bit in regards to the editing? I’m trying to be more conscious of this stuff, but I sometimes need it specifically pointed out to me…

WMDKitty@1

leap years get real weird on the larger scale, here’s what I found:

https://en.wikipedia.org/wiki/Leap_year#Gregorian_calendar

Intransitive says

The constant jump cuts and unnatural speech patterns instead of smooth edits. Think of it flashing lights to a person with Photosensitive Epilepsy.

abbeycadabra says

Leap years are not “every fourth year”. They’re ‘every fourth year, but not every 100th, unless it is also 400th’… actually more than that, but that covers the vast majority. 2000 wouldn’t have been a leap year except it is also divisible by 400.

Intransitive says

Yup. Because a year is not 365.25 days. It’s actually 365.2422, so leaving out the non-400 centuries is just enough days to smooth it out. It’s less than a day of error every thousand years.

Although microsoft’s excel still says it’s a leap year.

lochaber says

Intransitive, thanks for the explanation, I appreciate it.

Microsoft Excel… oh, the spreadsheets I have built just to try and prevent/correct the built-in “auto correct” and other automatic functions…

xohjoh2n says

Err, what just happened there ^^^^ I see 5 comments rolled into each other under #1 followed by the last @lochaber comment as #2, which I’m sure wasn’t always true…