In Leap Year's an extra Intercalary Day (Day 000) called 'Leap Year's Day' is inserted at the start of the year before 'New Year's Day'
Leap Year's Day, being an intercalary day, is Day 0 of Month 0
(New Year's Day is Day 1 of Month 0)
So, Leap Year's Day becomes a 6th Intercalary Day and makes Leap Years 366 Days long
By inserting Leap Year's Day at the START of the year and making it 'Day Zero' we keep the rest of the UCC Year perennial!
Leap Years follow a system of 8 Leap Years in a 33 Year Cycle, similar to the Persian Leap Year Cycle
8 / 33 = 0.2424 and so gives the most accurate 'rounding up' - closest to the 0.2422 of a day fraction left over each year
In this cycle years 1, 5, 9, 13, 17, 22, 26 & 30 are Leap Years as shown in RED in the table below... as are the Gregorian leap years
The current year and it's number in the 33 year cycle is highlighted in Yellow
Year UCC Year Gregorian Year UCC Year Gregorian Year 1 13476 1975/76 13509 08/09 2 13477 76/77 13510 09/10 3 13478 77/78 13511 2010/11 4 13479 78/79 13512 11/12 5 13480 79/80 13513 12/13 6 13481 1980/81 13514 13/14 7 13482 81/82 13515 14/15 8 13483 82/83 13516 2015/16 9 13484 83/84 13517 16/17 10 13485 84/85 13518 17/18 11 13486 85/86 13519 18/19 12 13487 86/87 13520 19/20 13 13488 87/88 13521 2020/21 14 13489 88/89 13522 21/22 15 13490 89/90 13523 22/23 16 13491 1990/91 13524 23/24 17 13492 91/92 13525 24/25 18 13493 92/93 13526 25/26 19 13494 93/94 13527 26/27 20 13495 94/95 13528 27/28 21 13496 95/96 13529 28/29 22 13497 96/97 13530 29/30 23 13498 97/98 13531 2030/31 24 13499 98/99 13532 31/32 25 13500 99/2000 13533 32/33 26 13501 2000/01 13534 33/34 27 13502 01/02 13535 34/35 28 13503 02/03 13536 35/36 29 13504 03/04 13537 36/37 30 13505 04/05 13538 37/38 31 13506 05/06 13539 38/39 32 13507 06/07 13540 39/40 33 13508 07/08 13541 2040/41