What is the ruling if a person's fast, based on conjecture, coincides with the month preceding Ramadan?

Primary text

If the fast coincides with the month preceding Ramadan, it does not suffice according to the majority of jurists, because he performed the act of worship before its prescribed time, similar to praying in cloudy weather before its time is established. This is distinguished from Hajj, where discrepancy might be excused for collective hardship.