Erroneous assumptions about the time of dawn and sunset
What is the ruling on erroneous assumptions about the time of dawn and sunset with regards to fasting?
According to the majority of scholars, making up a missed fast-day is obligatory if one eats after dawn thinking that dawn has not entered or eats before sunset presuming that the sun has set and finds out later that he was mistaken. Presumptions which prove to be erroneous are of no legal consequence.
In Al-Sunan Al-Kubra, Al-Bayhaqi reported through Shu'aib Ibn 'Amr Ibn Sulaym Al-Ansari who said, "My father and I broke our fast in the home of Suhayb Al-Khayr [the generous] in Ramadan on a drizzling cloudy day. Suddenly the sun appeared, so Suhaib said, ‘God has fed us. Complete your fast until sunset and make up a day in its place.' "