If a person is confined and unsure of the start or end of Ramadan, what is the ruling if he fasts based on conjecture?
General Chapter
Al-Mughni
Book of Fasting
Primary text
If a captive cannot ascertain the months and fasts based on a strong presumption of Ramadan's entry, his fast is valid if it coincides with Ramadan or the month following it, as he has fulfilled his duty through Ijtihad where evidence was unavailable. This is analogous to praying based on conjecture when the Qibla is uncertain, or praying in cloudy weather when the time is uncertain.
Supporting text
If it coincides with the month preceding Ramadan, it is not valid according to the majority, as he performed the act of worship before its time. Some Shafi'i scholars allow it if it matches the period between the two moons, analogous to Arafah. The difference is that Hajj is only excused for widespread hardship, not for a few individuals.