Helios calendar

Time on Earth has little meaning to someone who spends their life on another planet. While Martian calendars and clocks have existed since the 20th century, coordinating calendars for every last rock is untenable. In addition, compared to an observer outside of any gravitational well, a second on Earth is, in actuality, only .999999901895427 of a second.

The following was devised as a solution, at least in the context of the Solar System. When humanity begins taking the stars no doubt a galactic calendar will eventually be developed.

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The Solar Calendar is fairly simple, with only one unit. It defines the Solar Month as being one full rotation of the Sun at its equator. This is about 25 days, 9 hours, 7 minutes and 12 seconds. The exact value (2,192,832 seconds) is used rather than taking the minor variations in the Sun's rotation into account, of course. Time is tracked on the sole basis of the sun's rotation and thus, the Solar Calendar does not proceed at exactly the same rate Terran time does.

The epoch of the Solar Calendar begins with the legendary June 15th, 763 B.C.E solar eclipse during its maximum at 08:23 GMT. According to Roman legend, Remus was conceived during this eclipse, and it is mentioned in an Assyrian tablet known as the Eponym Canon. It is perhaps the most familiar as the eclipse referred to in Amos 8: "And it shall come to pass in that day, saith the Lord GOD, that I will cause the sun to go down at noon, and I will darken the earth in the clear day." (KJV)

Thus the 42,633rd Solar Month occurs about 20 days before 2201's New Year on Earth. Or simply, it's called the 633rd for short.

January 1st, Midnight GMT, 2201 is 42,633 Solar Months, 20 days, 5 hours, 12 minutes, and 16 seconds after the begining of the epoch. The additional ~9,172 seconds (if you bothered with the math) is due to relativity - Earth experiences time about a tenth of a millionth slower than an object in 'static' space around meaningfully outside any gravitational well.

February 22nd, Midnight GMT, 2222 is 42,938 Solar Months, 1 day, 7 hours, 37 minutes, and 21 seconds after the beginning of the epoch.

The 40,000th month began on December 18th, 2017 at 5:59:34 AM GMT.