This calendar is used by the Holy Protectorate of Sol and is a continuation of the Gregorian Calendar with a different origin point and axis.
Jan 1 1100 ED is 19 June 1630 AD. This date was selected by Lunar Administrator Thomas Shalan as the date that John Winthrop gave his speech about a city on the hill, and using advanced calculations he said had been revealed to him by the Christian God, he set the chart to go down, such that Dec 31 1100 rolls over to Jan 1 1099.
As such, it is usually out of sync on leap years from the AD calendar. And only the Protectorate has used it, even though with the only major colony being on the Moon, it doesn’t even fit their local calendar.