The calendar system utilized by most of the world is a confusing and irregular collection of historical inheritances mish-mashed together and with modifications and further exceptions being added to conform with contemporary technological uses. This article proposes a different sort of calendar, arguably better, for the consideration of the reader. One may note that it (a solar calendar) is specifically tailored to the Terran year, but there is also a section at the end discussing its suitability to be adapted for other locales in this solar system and also locales in other star systems.
In brief: a solar calendar taking the solar day as the basic unit, but with each day beginning at a specified convention for dawn (something like 5am according to current time-keeping) with the previous day ending at the same time (rather than midnight).
Going down, each day divided into 24 equal hours, or 1440 equal minutes.
Going up, 10 days per week (or ten-day), 3 weeks per month, 9 weeks per season, with 1-2 days of non-week, non-month, non-season holiday between the seasons.
So, a normal year will have one day each for the winter solstice and the two equinoxes, which are not considered part of the seasons and their months and weeks, with two days given over thus to the summer solstice. Leap years will be accounted for by adding an extra day to the winter solstice holiday every four years.
Roughly, that is. The figure used here for the Terran solar year is 365.242188972 days, or 24.2188792 extra days every hundred years (after the 365 standard days per year), so there will be one extra day every four years except for the hundredth year, and every five hundred years that centennial leap year keeps the extra day. And so on.
A year will then have either 365 or 366 days. The pattern will be: 365 [or 366] = 2-90-1-90-1 [or 2]-90-1-90.
Note that this places the solstice/equinox holidays between the seasons, instead of placing them mid-season, but that permits the usage of having the extra, and configurable, days between perfectly regular seasons and months and weeks. Every season has 90 days, every month has 30 days, and every week has 10 days (though if one considers a 10 day week too long, a 5 day week could be easily substituted in, for 6 weeks per month instead of 3, or 18 weeks per season instead of 9).
For myself I consider beginning the year with the summer solstice to be best, but a year could be designated to begin on any of the between-season holidays (such as the winter solstice, which would be more closely conformed to the Gregorian calendar in current standard use). For that matter the extra leap-year day could be added to the summer solstice holiday or one of the equinox holidays instead of to the winter solstice holiday.
The months could also be done away with entirely, to remove a layer of complexity. In that case dates would utilize the season instead of the month. For example:
3.4.autumn.865 (3rd day of the 4th week of autumn in the year 865)
65.spring.422 (65th day of spring in the year 422)
One could also choose to rename the four seasons—as something like primus (first, in Latin), secundus, tertius, quartus, or with whatever other nomenclature. But in any case the example dates offered above could also be rendered as:
3.4.II.865 (3rd day of the 4th week of autumn [2nd season] in the year 865)
65.IV.422 (65th day of spring [4th season] in the year 422).
Now, that given thus far is earth-centric, but does provide for regularized standards for time-keeping that are comfortably within human norms (regularized). Other orbital bodies within the solar system, once settled, could easily utilize the same calendar as presumably at least for some time Terran norms will continue to set the standard.
Alternatively, and for extra-solar settlement or exploration, the same pattern may be utilized for any conceivable local calendar needs. That is, take a basic unit such as the Terran solar day, go down to make divisions into hours and minutes of the 'day', and go up to make regular divisions of whatever the local year may be. Obviously this will be easier the closer those distinctions are to Terran norms, but the simple pattern of four equal divisions of a year with holidays in between should work everywhere. The holidays can be extended for odd numbers of days, or the seasons can be extended or shortened for different year-lengths, but the pattern will remain applicable. For extreme cases such as very short or long days, very short or long years, divisions or multiplications may be utilized. Every planet or orbital body will still have aphelion and perihelion, though the difference may be minute. A perfectly spherical orbit (though very unlikely) can have arbitrary seasons specified.
[Joseph Jones, 31 October 2021]