A perennial solar calendar based on astronomical observations.
The calendar year consists of 360 calendar days + 5 or 6 intercalary days. It is divided into 4 equal quarters of 1 intercalary day + 90 calendar days. The remaining 1 or 2 days of the year are intercalary transition days between years.
The 360 calendar days may be divided into 8 months of 45 days, as well as 40 weeks of 9 days (3 × 3, based on tridays). Alternatively, one can use the more traditional 12 months of 30 days and a 7-day week. A zero-indexed variant based on the 9-day week is also possible. See Variants for more.
The calendar year begins at the midnight closest to the instant of the northward equinox as measured from the prime meridian. Consequently, if the northward equinox falls before solar noon on a particular day, then that day is the first day of the year. If the northward equinox occurs after solar noon, the following day begins the calendar year.
The calendar may be used with any epoch. The proposed epoch is the beginning of the human era (10001 BC).
Essentially, it's the Persian calendar with a different epoch, a different meridian and a different division of the year. The calendar and its astronomical basis is deeply indebted to Persian astronomer Omar Khayyam's 11th century reform of the Jalali calendar.
For more details, see the calendar's project page.
Advantages[]
- Accurate — follows the true solar year
- Balanced – division of the year into equal parts
- Dynamic — allows for different variants and uses
- Predictable — has a consistent, perennial structure
- Simple — easy to learn and uncomplicated to use
It is structured, yet flexible enough to adapt to different uses and cultures:
- Agriculture — follows natural cycles
- Business — divided into equal parts, allows for flexible schedules
- Civil — simple and predictable
The calendar is not tied to any culture/religion, except inevitably to that of the current scientific paradigm. While it is scientifically grounded, it does not oppose combination with cultural or religious concepts.
Disadvantages[]
- Unfamiliarity — new divisions, units and beginning of year
- No simple leap year rule — a tradeoff for astronomical accuracy over time
- Yet another calendar — made by some commoner named Joakim (who is not the pope)
Variants[]
The foundation of this calendar enables several different implementations.
Traditional 12-month variant[]
This variant maintains the traditional 7-day week and 12 months, each month having 30 days. The months are perennial, but the weekdays are not.
Intercalary days belong to the seasons (named A-D) and the transition period (named X).
Months are offset from those of the Gregorian calendar, as new year is around March 20.
| Season | Month | Days | Gregorian | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| A | 00 | ||||||||||||||||
| 1 | 01 | 02 | 03 | 04 | 05 | 06 | 07 | 3 - 4 | |||||||||
| 08 | 09 | 10 | 11 | 12 | 13 | 14 | |||||||||||
| 15 | 16 | 17 | 18 | 19 | 20 | 21 | |||||||||||
| 22 | 23 | 24 | 25 | 26 | 27 | 28 | |||||||||||
| 29 | 30 | ||||||||||||||||
| 2 | 01 | 02 | 03 | 04 | 05 | 4 - 5 | |||||||||||
| 06 | 07 | 08 | 09 | 10 | 11 | 12 | |||||||||||
| 13 | 14 | 15 | 16 | 17 | 18 | 19 | |||||||||||
| 20 | 21 | 22 | 23 | 24 | 25 | 26 | |||||||||||
| 27 | 28 | 29 | 30 | ||||||||||||||
| 3 | 01 | 02 | 03 | 5 - 6 | |||||||||||||
| 04 | 05 | 06 | 07 | 08 | 09 | 10 | |||||||||||
| 11 | 12 | 13 | 14 | 15 | 16 | 17 | |||||||||||
| 18 | 19 | 20 | 21 | 22 | 23 | 24 | |||||||||||
| 25 | 26 | 27 | 28 | 29 | 30 | ||||||||||||
| B | 00 | ||||||||||||||||
| 4 | 01 | 02 | 03 | 04 | 05 | 06 | 07 | 6 - 7 | |||||||||
| 08 | 09 | 10 | 11 | 12 | 13 | 14 | |||||||||||
| 15 | 16 | 17 | 18 | 19 | 20 | 21 | |||||||||||
| 22 | 23 | 24 | 25 | 26 | 27 | 28 | |||||||||||
| 29 | 30 | ||||||||||||||||
| 5 | 01 | 02 | 03 | 04 | 05 | 7 - 8 | |||||||||||
| 06 | 07 | 08 | 09 | 10 | 11 | 12 | |||||||||||
| 13 | 14 | 15 | 16 | 17 | 18 | 19 | |||||||||||
| 20 | 21 | 22 | 23 | 24 | 25 | 26 | |||||||||||
| 27 | 28 | 29 | 30 | ||||||||||||||
| 6 | 01 | 02 | 03 | 8 - 9 | |||||||||||||
| 04 | 05 | 06 | 07 | 08 | 09 | 10 | |||||||||||
| 11 | 12 | 13 | 14 | 15 | 16 | 17 | |||||||||||
| 18 | 19 | 20 | 21 | 22 | 23 | 24 | |||||||||||
| 25 | 26 | 27 | 28 | 29 | 30 | ||||||||||||
| C | 00 | ||||||||||||||||
| 7 | 01 | 02 | 03 | 04 | 05 | 06 | 07 | 9 - 10 | |||||||||
| 08 | 09 | 10 | 11 | 12 | 13 | 14 | |||||||||||
| 15 | 16 | 17 | 18 | 19 | 20 | 21 | |||||||||||
| 22 | 23 | 24 | 25 | 26 | 27 | 28 | |||||||||||
| 29 | 30 | ||||||||||||||||
| 8 | 01 | 02 | 03 | 04 | 05 | 10 - 11 | |||||||||||
| 06 | 07 | 08 | 09 | 10 | 11 | 12 | |||||||||||
| 13 | 14 | 15 | 16 | 17 | 18 | 19 | |||||||||||
| 20 | 21 | 22 | 23 | 24 | 25 | 26 | |||||||||||
| 27 | 28 | 29 | 30 | ||||||||||||||
| 9 | 01 | 02 | 03 | 11 - 12 | |||||||||||||
| 04 | 05 | 06 | 07 | 08 | 09 | 10 | |||||||||||
| 11 | 12 | 13 | 14 | 15 | 16 | 17 | |||||||||||
| 18 | 19 | 20 | 21 | 22 | 23 | 24 | |||||||||||
| 25 | 26 | 27 | 28 | 29 | 30 | ||||||||||||
| D | 00 | ||||||||||||||||
| 10 | 01 | 02 | 03 | 04 | 05 | 06 | 07 | 12 - 1 | |||||||||
| 08 | 09 | 10 | 11 | 12 | 13 | 14 | |||||||||||
| 15 | 16 | 17 | 18 | 19 | 20 | 21 | |||||||||||
| 22 | 23 | 24 | 25 | 26 | 27 | 28 | |||||||||||
| 29 | 30 | ||||||||||||||||
| 11 | 01 | 02 | 03 | 04 | 05 | 1 - 2 | |||||||||||
| 06 | 07 | 08 | 09 | 10 | 11 | 12 | |||||||||||
| 13 | 14 | 15 | 16 | 17 | 18 | 19 | |||||||||||
| 20 | 21 | 22 | 23 | 24 | 25 | 26 | |||||||||||
| 27 | 28 | 29 | 30 | ||||||||||||||
| 12 | 01 | 02 | 03 | 2 - 3 | |||||||||||||
| 04 | 05 | 06 | 07 | 08 | 09 | 10 | |||||||||||
| 11 | 12 | 13 | 14 | 15 | 16 | 17 | |||||||||||
| 18 | 19 | 20 | 21 | 22 | 23 | 24 | |||||||||||
| 25 | 26 | 27 | 28 | 29 | 30 | ||||||||||||
| X | 00 | ||||||||||||||||
| 01 | in leap years | ||||||||||||||||
Proposed 8-month variant[]
This variant has 8 octants of 45 days each, grouped into nonads of 9 days each.
Intercalary days belong to the seasons (named A-D) and the transition period (named X).
| Season | Octant | Days | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| A | 00 | |||||||||
| ☳ | 1st | 01 | 02 | 03 | 04 | 05 | 06 | 07 | 08 | 09 |
| 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | ||
| 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | ||
| 28 | 29 | 30 | 31 | 32 | 33 | 34 | 35 | 36 | ||
| 37 | 38 | 39 | 40 | 41 | 42 | 43 | 44 | 45 | ||
| ☴ | 2nd | 01 | 02 | 03 | 04 | 05 | 06 | 07 | 08 | 09 |
| 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | ||
| 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | ||
| 28 | 29 | 30 | 31 | 32 | 33 | 34 | 35 | 36 | ||
| 37 | 38 | 39 | 40 | 41 | 42 | 43 | 44 | 45 | ||
| B | 00 | |||||||||
| ☲ | 3rd | 01 | 02 | 03 | 04 | 05 | 06 | 07 | 08 | 09 |
| 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | ||
| 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | ||
| 28 | 29 | 30 | 31 | 32 | 33 | 34 | 35 | 36 | ||
| 37 | 38 | 39 | 40 | 41 | 42 | 43 | 44 | 45 | ||
| ☷ | 4th | 01 | 02 | 03 | 04 | 05 | 06 | 07 | 08 | 09 |
| 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | ||
| 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | ||
| 28 | 29 | 30 | 31 | 32 | 33 | 34 | 35 | 36 | ||
| 37 | 38 | 39 | 40 | 41 | 42 | 43 | 44 | 45 | ||
| C | 00 | |||||||||
| ☱ | 5th | 01 | 02 | 03 | 04 | 05 | 06 | 07 | 08 | 09 |
| 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | ||
| 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | ||
| 28 | 29 | 30 | 31 | 32 | 33 | 34 | 35 | 36 | ||
| 37 | 38 | 39 | 40 | 41 | 42 | 43 | 44 | 45 | ||
| ☰ | 6th | 01 | 02 | 03 | 04 | 05 | 06 | 07 | 08 | 09 |
| 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | ||
| 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | ||
| 28 | 29 | 30 | 31 | 32 | 33 | 34 | 35 | 36 | ||
| 37 | 38 | 39 | 40 | 41 | 42 | 43 | 44 | 45 | ||
| D | 00 | |||||||||
| ☵ | 7th | 01 | 02 | 03 | 04 | 05 | 06 | 07 | 08 | 09 |
| 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | ||
| 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | ||
| 28 | 29 | 30 | 31 | 32 | 33 | 34 | 35 | 36 | ||
| 37 | 38 | 39 | 40 | 41 | 42 | 43 | 44 | 45 | ||
| ☶ | 8th | 01 | 02 | 03 | 04 | 05 | 06 | 07 | 08 | 09 |
| 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | ||
| 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | ||
| 28 | 29 | 30 | 31 | 32 | 33 | 34 | 35 | 36 | ||
| 37 | 38 | 39 | 40 | 41 | 42 | 43 | 44 | 45 | ||
| X | 00 | 01 | ||||||||
Zero-indexed variant[]
This variant is an even greater departure from traditional calendars. Although fairly simple, it is likely harder to grasp due to its unfamiliarity. Especially the concept of zero, which is here used to denote holidays at the start of a season.
A calendar year has 4 seasons, each having 10 sets (a grouping of days). Years, seasons, sets and holidays are zero-indexed, while nonads (9 days) and their workdays are one-indexed.
The days of the zeroth set of a season are holidays. The remaining 9 sets are ordinary nonads. That makes for a total of 36 nonads, or 324 workdays, per year. Using the concept of tridays, a person may work ⅔ of those (216 days per year), with 10-day seasonal holidays each quarter.
In this variant, the transition period is placed at the start of the year. Conceptually, it is between years, but every day must have a date. Here it belongs to the year it introduces, becoming the zeroth season.
| Season | Set | Days | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 – Transition | 0 | 0 | 1 | ||||||||
| 1 – Spring | 0 | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
| 1 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||
| 2 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||
| 3 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||
| 4 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||
| 5 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||
| 6 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||
| 7 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||
| 8 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||
| 9 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||
| 2 – Summer | 0 | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
| 1 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||
| 2 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||
| 3 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||
| 4 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||
| 5 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||
| 6 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||
| 7 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||
| 8 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||
| 9 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||
| 3 – Autumn | 0 | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
| 1 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||
| 2 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||
| 3 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||
| 4 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||
| 5 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||
| 6 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||
| 7 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||
| 8 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||
| 9 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||
| 4 – Winter | 0 | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
| 1 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||
| 2 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||
| 3 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||
| 4 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||
| 5 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||
| 6 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||
| 7 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||
| 8 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||
| 9 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||
In these examples, seasons are named in English for the northern hemisphere. While the numbers are universal, the names will vary by hemisphere and language.
Dates are expressed using numbers and interpunct: year · season · set · day. Season, set and day are single-digit. For example (2020-09-17 Gregorian): 2020·2·9·9. In the northern hemisphere, 2 · 9 · 9 tells you it's the 2nd season of the year, meaning summer. It is the last set (9) of that season, and therefore the last day (9) of summer. Autumn is upon us, and winter is coming.
More dates:
- 2020·0·0·0 – Transition day
- 2020·0·0·1 – Leap day (2020 was a leap year)
- 2020·1·0·0 – New year's day, and the zeroth day of spring holidays
- 2020·1·0·1 – First day of spring holidays
- 2020·1·1·1 – First workday after spring holidays
- 2020·2·4·9 – Halfway point of the summer season (at noon)
- 2020·2·5·1 – The following day (there is no 2·5·0)
- 2020·4·9·9 – Last day of the year, and of the winter season
Any date with a zero in it is a holiday.
An interesting side effect is that the year can be rendered as a circle of 400 gradians, where the "skipped" zero-days function as separators between nonads.
In the 12-month and 8-month variants, a circle of 360 degrees can represent all calendar days.
Questions and Answers[]
Why base it on the Persian calendar?[]
It is the most accurate solar calendar designed. I'd also argue that it is the most elegant. Tying the calendar to astronomical observations of the northward equinox is not only scientifically sound, but on some level even poetic. Omar Khayyam was after all both an astronomer and a poet.
Why does the calendar year begin at the northward equinox?[]
The year being reborn in spring is an idea that resonates deeply with people in many cultures. We have a long history of celebrating the coming of spring around northward equinox (for example Nowruz, Holi, Falles, Marzanna, Dísablót, Teotihuacán, Angkor Wat and various South/Southeast Asian solar New Year traditions) — often symbolizing rebirth or fertility, often involving bonfires or burning of effigies.
(At least in the northern hemisphere, where approximately 90% of us live. Apologies to the good people of the southern hemisphere.)
Why have intercalary quarter days?[]
360 calendar days leaves 5 (or 6 in leap years) intercalary days. Having 4 equal quarters representing the 4 astronomical seasons enables the intercalary days to be distributed throughout the year, instead of lumping them all together in a mini-period at the end.
This also makes the length of a quarter as close as possible to 365 / 4.
Why place the extra day(s) at the end of the year?[]
After 4 quarters have passed, the remaining one or two days before the next calendar year begins are intercalary transition days, representing the transition from the old year to the new. The cultural significance of this may vary. For example, it could be a time for reflection and forgiveness.
Having the extra day(s) at the end of the year is also a consequence of this being an observational calendar. The first day of a calendar year is simply determined by when the northward equinox occurs. Placing the extra day(s) anywhere else in the year would greatly complicate the calendar.
Is the calendar secular? Religious?[]
It is absolutely neutral to this question. By being tied to the northward equinox, the calendar can be both scientifically precise and religiously relevant.
What's up with the name?[]
The calendar actually doesn't have a name. But it needed some sort of title, and A Calendar for Time to Come sounded good. It has several interpretations, all of which seemed fitting for an accurate calendar for the future.