Why does the year end in 4 a, 5 b and 6 c?

This problem should be said to be the reason why the Gregorian calendar and the heavenly calendar are both decimal. Gregorian calendar dates begin in the first year of AD, with natural numbers and decimal notation, while calendar dates use A, B, C, D and E (ten days). Although the methods are different, they can all be understood as a ten-year cycle. Therefore, when the mantissa of the Gregorian calendar year is 4, the ten-day calendar year happens to be a year.