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The Seth Calendar was invented by Seth Freiberg in 2026 as a personal design project, born out of frustration with the irregularity of the Gregorian calendar — unequal month lengths, weeks that don't divide months evenly, and no clean mapping between days and dates.

The goal was a calendar with a simple, regular structure: 10 months of exactly 36 days, each month divided into 6 weeks of 6 days, with a small block of holiday days at the year end to absorb the remainder. Year numbers and the January 1 new year are kept from the Gregorian calendar to stay compatible with the existing world.

Decimal time was paired with it for the same reason: the standard 24-hour clock is an arbitrary Babylonian inheritance. Dividing the day into 10 hours of 100 minutes of 100 seconds gives a fully base-10 time system that is easier to reason about and calculate with. All units in the Seth system — months, days, hours, minutes, seconds — are zero-indexed.

Credit is due to the French Republican Calendar (1793), which pioneered both ideas: a reformed calendar with equal 30-day months and 5–6 complementary days at year end, and a decimal clock dividing the day into 10 hours of 100 minutes of 100 seconds. The French system was officially used from 1793 to 1805 before Napoleon abolished it. The Seth Calendar was designed independently in the same spirit, with a different month structure and the addition of zero-indexing throughout.

Seth Date uses the same year numbers and January 1 new year as the Gregorian calendar. The year is divided into 10 months of 36 days each, followed by 5 auxiliary (intercalary) days at year end (H0-H4, always 5). All units are zero-indexed: months 0–9, days-of-month 0–35. Time uses the same decimal system: 10 hours, 100 minutes, 100 seconds per day. The Seth second is derived from the Unix second (which equals the SI second, leap seconds aside) at a fixed ratio: 1 Seth second = 0.864 SI seconds (86,400 SI seconds per day ÷ 100,000 Seth seconds per day).

Leap Day (Gregorian Feb 29) is a special intercalary day that exists outside the normal month structure. It is inserted at day-of-year 60, between day 22 and day 23 of Month 1. After Leap Day the calendar resumes unchanged, which is why every Seth date from March 1 onward falls on the same Gregorian date every year (e.g. Christmas is always Month 9, Day 34). On the calendar it appears as a split cell: the top half is day 22, the bottom half is Leap Day.

Date notation: dates are written as day.month (day-of-month first, zero-indexed).

Format	Pattern	Example (day 16 of month 3, year 2026)
Short	D.M	16.3
Medium	D.M.YY	16.3.26
Log	DD.M.YYYY	16.3.2026
Full / reverse	YYYY.M.DD	2026.3.16

Auxiliary days are written as H0–H4 (always 5 total).

Months (months 0–9, days 0–35, approx. Gregorian ranges):

#	Days	Approx. Gregorian	Notes
0	0–35	Jan 1 – Feb 5
1	0–35	Feb 6 – Mar 13
2	0–35	Mar 14 – Apr 18
3	0–35	Apr 19 – May 24
4	0–35	May 25 – Jun 29
5	0–35	Jun 30 – Aug 4
6	0–35	Aug 5 – Sep 9
7	0–35	Sep 10 – Oct 15
8	0–35	Oct 16 – Nov 20
9	0–35	Nov 21 – Dec 26	Ends Dec 25 on leap years

Auxiliary days (after Month 9, Day 35 — always exactly 5):

Holiday	Normal year	Leap year
H0	Dec 27	Dec 27
H1	Dec 28	Dec 28
H2	Dec 29	Dec 29
H3	Dec 30	Dec 30
H4	Dec 31 — New Year's Eve	Dec 31 — New Year's Eve

Leap Day (Feb 29) is the intercalary day, not an extra holiday.
Christmas (Dec 25) falls on Month 9, Day 34 in both normal and leap years.
Dec 26 (Boxing Day) falls on Month 9, Day 35 in both normal and leap years.

Each decimal hour = 2 hours 24 minutes Gregorian. Each Gregorian hour = ~0:41:67 decimal.