Is there a difference in validity between counting the two months by the lunar appearance (*Ahillah*) versus counting exactly sixty days?
General Chapter
Al-Mughni
Book of Zihar
Primary text
It is permissible to begin the two months of consecutive fasting from the beginning of a lunar month or in the middle of a month; there is no known dispute on this. If one starts from the beginning of a month and fasts two full lunar months, this is sufficient, whether those months are complete (30 days) or deficient (29 days), by consensus (*Ijma'*). This is the position of Al-Thawri, the people of Iraq, Malik among the people of Hijaz, Al-Shafi'i, Abu Thawr, and Abu Ubayd. Furthermore, if one starts in the middle of a month and fasts exactly sixty days, this is also sufficient without dispute. Ibn al-Mundhir reports consensus on this from those scholars whose views he recorded.
Supporting text
If one fasts one month by the lunar cycle and another by exact count (e.g., 15 days of Muharram, all of Safar, and 15 days of Rabi'), this is sufficient, provided Safar is counted as 30 days regardless of its actual length. This is the position of Al-Shafi'i and the Ashab al-Ra'y, as the default is to follow the lunar appearance, which is only abandoned when starting mid-month. However, it is argued that only two months counted by exact days should suffice, because by completing the first month (15 + 15 = 30 days), the second month also begins mid-cycle, making the entire period based on exact count, which is the view of Al-Zuhri.