SAC13 • SAC13 vs others


S	SAC13
G	Gregorian Calendar
1	Symmetry454
2	International Fixed Calendar
3	World Calendar
4	Hanke–Henry Permanent Calendar

Feature	S	G	1	2	3	4
days per week	7⁺	7	7	7⁺	7⁺	7
months per year	13	12	12	13	12	12⁺
leap days	•	•		•	•
leap weeks			•			•
leap months						•
fixed layout	✅	❌	✅	✅	✅	✅
fixed months per year	✅	✅	✅	✅	✅	❌
whole week months	✅	❌	✅	✅	❌	✅
No “blank” days	✅	✅	✅	❌	❌	✅
all years same length*	✅	✅	❌	✅	✅	❌
not redefining weekdays ʳ	✅	✅	✅	❌	❌	❌
strict week ʷ	❌	✅	✅	❌	❌	✅
whole month quarters	❌	✅	✅	❌	✅	❌
week starts Monday	⭕	⭕	✅	❌	❌	✅
equal* months	✅	❌	❌	✅	✅	❌
equal* quarters	✅	❌	❌	✅	✅	❌
whole week quarters	✅	❌	✅	✅	❌	❌
less calendar drift	✅	❌	✅	❌	❌	❌
reference source ¹	✅	⭕	⭕	❌	❌	❌
Documentation ²	✅	✅	✅	❌	❌	❌
same last date ³	✅	✅	❌	✅	✅	❌
Well defined limits	✅	❌	❌	❌	❌	❌
meaningful new year	✅	❌	❌	❌	❌	❌
Sept=7, Oct=8, etc..	✅	❌	❌	❌	❌	❌
No negative years	✅	❌	❌	❌	❌	❌

* one day difference is allowed, because it would otherwise be practically impossible for any reasonable calendar because of leap days.
⁺ Typically, but on leap years one more
ʳ Doesn’t use weekday names from the Gregorian Calendar in an incompatible way
ʷ religiously strict week, also probably simplifies parallel use with Gregorian Calendar
¹ Official, permissive, open source, reference implementation
² Detailed “official” documentation about algorithms like leap years, date calculations, etc.
³ Each year (common year or leap year) ends on the same date. Gregorian yes, but weekday not fixed.

I intentionally ignored calendar proposals with week lengths that are not seven days, here is why.

Changing to a different calendar system is always a massive undertaking (if even possible in our modern world), so we should fix as many issues as possible with “the next version”. The cost of switching to a new calendar system is big (details here), but pretty much constant no matter to which system. So it doesn’t really make sense try to switch to a calendar, that tries to just apply a very small patch to the Gregorian Calendar and leaves a lot of other unfixed issues behind, because it would cost the same to only fix a small thing.