Motivation

Here are some of the issues I have with the Gregorian Calendar that led me to work on this proposal:

It’s not a perennial calendar

Neither the length of a common year (365 days) nor a leap year (366 days) are divisible by seven, because of that the days of the week are not fixed with regard to the day of the month. Because of that, there are actually 14 different year layouts in the Gregorian Calendar. If I’d ask you which day of the week the 14^th June 2027 is, chances are you’ll have no idea. There are methods to do that calculation in your head (for example Doomsday rule) but it’s certainly not easy. Weeks are technically not even part of the Gregorian Calendar but are actually overlaid onto it. SAC13 fixes that by using one eight-day week a year (two on leap years). For details see SAC13 week.

The start of the year is completely arbitrary

There are people who think that New Year happens at the end of December because of the winter solstice, but that’s not true. December solstice typically happens around the 21^st so about 10 days before New Year. For a very long time the year started with March, until the Romans changed it for political and tactical reasons in 153 BC. A modern calendar shouldn’t start the year at a practically random point just because Roman consuls took office around that time of year.

The start of the year should be deliberate and something important for most cultures and that’s why SAC13 starts the year with the March equinox. The start of the year doesn’t have to be the March equinox to be a good calendar, but there should be good and valid reasons for picking that specific point in time to be the start of the year.

The year is misaligned with the seasons

Follow up from the previous issue.
Over the course of a year there are several interesting points the Earth passes on its orbit around the sun. March equinox, June solstice, September equinox and the December solstice for example mark the start and end of the astronomical seasons.

Because the December solstice is currently around of December 21 and the next year begins 10 days later, the Gregorian Calendar is misaligned by roughly 10 days with the astronomical seasons. This leads to a situation where a lot of data and diagrams are ever so slightly shifted and asymmetrical.

The days per month are all over the place

31, 28 or 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 doesn’t really make a lot of sense. Many people (including me) couldn’t even tell you how many days “September” has, without knuckle counting (Wikipedia).

The names of the months are shifted

The last four months got their name from Latin: September (septem, seven), October (octo, eight), November (novem, nine), December (decem, ten). But those numbers no longer reflect the positions they are in. Originally those months were supposedly part of a ten month Roman calendar when they switched to a twelve month calendar they added two months at the bottom (doesn’t cause any issues), but political changes in 153 BC caused more and more people to see January as the start of the year, which caused those numbered months to be in the wrong place.

Calendar drift

The leap year rules for the Gregorian Calendar average to a year length of 365.2425 days, but in reality it’s a bit shorter than that, which causes the Gregorian Calendar to drift ever so slightly.

To be fair, in practice that’s probably a small issue, because the current drift is slow enough, that we can plan centuries ahead of time to skip specific leap years. For example we could decide (and slowly update todays software) that the year 2600 won’t be a leap year (I just picked that year at random). We could keep everything the same and we’d have about 600 years to make sure all our systems know about that exception, which would certainly be more than enough given the fact that we already have and do solve other time-related issues all the time.

We could also just accept, that the calendar is drifting, because it’s so slow that it might not even be an issue at all for future generations on to have winter in the northern hemisphere in July.

But!, because we already change the calendar, so why not fix that too?

Shifted millennia

Because the Gregorian Calendar doesn’t have a year zero, the millennia are offset by one year with respect to the thousands place of the year. The year 1999 is part of the 2nd millennium (AD), 2000 was also in the second millennium and and 2001 was the start of the 3rd millennium. More about how SAC13 fixes that here: SAC13 year.