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The real innovation in this calendar, which I don't rememebr seeing in previous perpetual calendar proposals, is the system for 7-day-week seasonal drift adjustment. Adding an "extra" 5 days to every year is much more common, but not popular.

What I can't figure out is what algorithm they use to calculate when to add an extra week. It seems slightly irregular, but I asssume there must be a simple formula I could use in a date library to account for it.

Well, with 364 days that means there's 1.25 days pushed into the buffer every year. That means that the buffer would be full (mathematically) every 5.6 years hence the "every five or six years." In order to keep general season alignment, I figure they vary between the two. This decimal doesn't go away until after 28 years - so I assume some combination of fives and sixes that add up to 28. Lets say three sixes and two fives. (18+10). So in order to make any pattern they might do something like 6 5 6 5 6 6 5 6 5 6. And now, looking at the leap years, I think my theory is confirmed: 2015-2020 (5) 2020-2026 (6) 2026-2032 (6) 2032-2037 (5) 2037-2043 (6) 2043-2048 (5) 2048-2054 (6) 2054-2060 (6)

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