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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, weeks, 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 (6 weeks of 6 days each), followed by 5 holiday days at year end (6 on leap years). All units are zero-indexed: months 0–9, days 0–35, weeks 0–5, weekdays 0–5. 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 and week structure. It is inserted between Month 1, Week 3, Day 4 and Month 1, Week 3, Day 5 — occupying DOY 60 in its own slot. After Leap Day the calendar resumes at D5 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 sharing the D4 column: the top half is the normal D4 day, the bottom half is Leap Day.

Reading the date: dates are written as Year M W D — e.g. Month 3, Week 2, Day 4 means the 3rd month (0-indexed), week 2, day 4 of that week, which is day 16 of the month (2×6 + 4). Holiday days are written as Holiday N (N = 0–4, or 0–5 on leap years) and fall outside any month or week.

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

Weeks (weeks 0–5 within each month, days 0–5 within each week):

Week	Days
0	0–5
1	6–11
2	12–17
3	18–23
4	24–29
5	30–35

Holiday days (after Month 9, Day 35):

Holiday	Normal year	Leap year
H0	Dec 27	Dec 26 — Boxing Day
H1	Dec 28	Dec 27
H2	Dec 29	Dec 28
H3	Dec 30	Dec 29
H4	Dec 31 — New Year's Eve	Dec 30
H5*	—	Dec 31 — New Year's Eve

* Leap years only.
Christmas (Dec 25) falls on Month 9, Day 34 in normal years, and Month 9, Day 35 in leap years.
Dec 26 (Boxing Day) is Month 9, Day 35 in normal years, and Holiday 0 in leap years.

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