Happy New Year's Day

[lpetrich]

In my timezone, it's the day before, but in Russia, Turkey, East Africa, and further east to the Internetional Date Line, it's already New Year's Day.

New Year's Day, History of calendars

It's a rather arbitrary temporal landmark, and different calendars have used different stars of the year. The original Roman one was March 1, near the vernal equinox, and some other calendars start the year around then. However, some calendars use the autumnal equinox, like the Jewish calendar and the French revolutionaries' calendar. Our New Year is close to the winter solstice, and the Chinese New Year is about halfway between the winter solstice and the vernal equinox. The Olympiads began in midsummer, around the summer solstice.

So it is evident that which day qualifies as New Year's Day is very arbitrary.

[lpetrich]

Now for which day it is in various calendars. I'll use 2018 January 1 in our calendar.

In the Julian calendar, Julius Caesar's original calendar, it is 2017 December 19, 13 days behind.

In the French Republican Calendar, it is 226 Nivôse 12.

In the Jewish calendar, it is 5778 Teveth 14.
The year starts on the autumnal equinox.

In the Muslim calendar, it is 1439 Rabi'-ath Thani 13.
The year is exactly 12 lunar months, and it gradually rotates relative to the seasons.

In the Iranian calendar, it is 1396 Dey 11.
The year starts on the vernal equinox.

In the Indian civil calendar, it is 1939 Pausa 11.
India has numerous traditional calendars, and some of them are still in use.

In the Mayan Long Count, it is 13.0.5.1.17 .
Haab (civil): 15 Kankin, Tzolkin (religious): 8 Caban.

This day is in the 1st year of the 699th Olympiad. Since each Olympiad is 4 years, we can define an "Olympic Year": 2793.

Using January 1 as start dates, this year is Seleucid year 2329, dating form when one of Alexander the Great's generals conquered Babylon, and AUC year 2770, dating from Marcus Terentius Varro's calculation of when Rome was founded.

Calendar calculator: Calendar Converter, Convert a date

[lpetrich]

Our own calendar's first year is from a certain Dionysius Exiguus around 525 CE. He tried to calculate which year Jesus Christ was born, though his calculation was a few years off. It was popularized by the Venerable Bede, who used it to date events in his Ecclesiastical History of the English People (731 CE).

The Jewish calendar is counted from an estimate of when the Universe was created according to the Bible. That requires adding up its numerous "begots" and making various assumptions to fill in various gaps, and Archbishop Ussher's figure of 4004 BCE is only one of numerous calculated values.

The values cluster around 4000 BCE for the Masoretic version of the Old Testament, and 5500 BCE for the Septuagint version -- the numbers in the begots are different.


I'll now turn to simplified calendar schemes, simplified because they use simple numbers instead of compound ones.

The Julian-Day system, used by astronomers, has date 2458119.5 for midnight GMT. A simplified version is the Modified Julian Day, obtained by subtracting 2400000.5. It gives 58119.

We also have computer-related conventions like:

Microsoft Excel: 43101 days.

Unix time() function: 1514764800 seconds.

[lpetrich]

More convention fun: days can be defined with astronomical landmarks as beginning at sunrise, at noon, at sunset, or at midnight -- and all four conventions have been used by various people.

Something like astronomical landmarks for the year -- the solstices and the equinoxes.


Astronomy on Mars, Timekeeping on Mars, Darian calendar, Mars' Calendar | The Planetary Society are about timekeeping on Mars and a proposed calendar for that planet.

The Planetary Society's link mentions a convention for New Year's Day on that planet: the northern-hemisphere vernal equinox. The most recent one was 2017 May 5, and the next one will be 2019 Mar 23.

[lpetrich]

Using the original system for designating years, the year-reign system, we get dates like
Donald Trump 2
Queen Elizabeth II 67 -- Theresa May 3, Justin Trudeau 4, Malcolm Turnbull 4, Jacinda Ardern 2
Emmanuel Macron 2
Frank-Walter Steinmeier 2, Angela Merkel 14
Vladimir Putin II-6
Emperor Akihito 29
Xi Jinping 7
Ram Nath Kovind 2, Narendra Modi 5
...