The Gregorian calendar is of the opinion that a ” calendaristic year” is that year which begins on January 1 and ends on December 31.
It can be said that the calendaristic year r begins on New Year’s Day and ends one day before the next first day of the following New Year. In the case of the Gregorian calendar, this cycle has 365 days in the normal years and 366 days when we have the bisected years.
The condition is that the calendaristic year is made up of a natural number of days.
In order to agree, the calendaristic year with the astronomical year, (which does not consist of an entire number of days), certain years contain so-called “extra days”. By introducing these time units, the calendaristic year can be synchronized with the astronomical year. Even so, these units of time are not considered as part of the calendar.