Presidential proclamations are signed and dated in two calendars, "the year of our Lord", and "the year of the Independence of the United States of America":
IN WITNESS WHEREOF, I have hereunto set my hand this twenty-fifth day of October, in the year of our Lord two thousand twenty-one, and of the Independence of the United States of America the two hundred and forty-sixth.
The former uses January 1st as the start of each new year; the latter, July 4th.
In England, Lady Day (March 25th) was the turnover of a new year, for about 6 centuries.
The point of these "falsehoods" threads is that programmers may not care about these niche anthropology facts, but users may care a lot.
It's not crazy to think that a user may, say, want to create a database of historical records, like when people were born, married or buried. Now I want to find out the age people were when married... suddenly, the existence of Lady Day and the time and place the dates were recorded become very important.
I think they mean that if someone says "remind me in a month"
On December 3rd, the reminder would be next year.
I've never met a developer who believed that, but I've met plenty who would forget about the edge-case and write a buggy time-library. As is the case for most of these.
I think they are talking about the End Of Period for a month. Whatever batch or closeout type processing happens during that time period can flow into the next year, despite it being flagged/tagged/marked with the last month of the prior year. So, be careful with using dates! (unless you are a Time Druid)
It could also be a reference to the start of the calendar, but that isn't bound to happen again.... Well, unless it does.
Then, on the other hand (because Time is funky that way) your time may not be the same as my time, since we could be in different time zones.
Okay, of course a week can begin in an year and end in the next, but how exactly would a month not begin and end in the same year?