Kurdish calendar by a leap year every fourth year. It starts with the exact first day of spring according to the Gregorian calendar (March 20 or 21). Like the Gregorian system, the Kurdish calendar divides the year into four seasons: Buhar, Tawistan or Hawín, Payiz and Zimistan. It divides the year into 12 months, each month into four weeks and every week into seven days. In the Kurdish calendar the first six months (comprising spring and summer) are each 31 days long, while the next five months (in fall and winter) are 30 days each. The last winter month, the 12th month
Gregorian calendar always obtained by doubling (the "bissextum" (twice sixth) or bissextile day) until the late Middle Ages. The Gregorian calendar is proleptic before 1582 (assumed to exist before 1582). The following equation gives the number of days (actually, dates) that the Gregorian calendar is ahead of the Julian calendar, called the "secular difference" between the two calendars. A negative difference means the Julian calendar is ahead of the Gregorian calendar. where formula_2 is the secular difference and formula_3 is the year using astronomical year numbering, that is, use for BC years. formula_4 means that if the result of the division is