In 1900, Maksim Trpković revised the #JulianCalendar. Under his system, an year that yields a remainder of 0 or 400 when divided by 900 is a #LeapYear.

Milutin Milanković slightly altered this in 1923, changing the rule to years that give a remainder of 200 or 600. A mean year in this calendar is just 2 seconds off the #TropicalYear. It is used by some, but not all, #EasternOrthodox churches.

In the 17th century, #IsaacNewton recognised that the #julianCalendar was deficient but the #GregorianCalendar was too #Catholic for him. He worked on a new calendar whose mean year was just a few seconds shorter than the #TropicalYear (the mean Gregorian year is about 29 seconds longer). Newton viewed this as a feature rather than a bug, because under his system, the dates of the solstices and equinoxes would gradually drift back to those when #Jesus was alive.

The Gregorian calendar came into force in 1568 and was promptly adopted in #Catholic countries. #Protestant and #Orthodox countries were slower, since they didn't want to use a #calendar invented by Catholics.

In 1568, 10 days were skippedin #October to recalibrate the civil calendar with the #TropicalYear. By the time #Britain gave in and adpted the Gregorian calendar in 1758, they had to sip 11 days.

But, as we now know, the #JulianCalendar's mean year is about 11 minutes 15 seconds longer than the #TropicalYear, and by the 4th century AD, the dates of the #solstices and #equinoxes had drifted from their official dates by several days.

In the 16th century, #Pope Gregory XIII used his authority as #PontifexMaximus to reform the calendar again, because that is something #popes can do.

This means that the Romans had no real concept of, say, "The 28th of Martius"; they thought of that day as "three days before the kalends of Aprilis".

But there was another issue. Even with 12 months, the normal #calendar year was a little shorter than the #TropicalYear. No problem, right? Just add an extra month every 2-3 years, like basically every other culture does?

Yeah, the Romans were *the worst* at timekeeping.