What is the ruling concerning erroneous assumptions about the time of dawn and sunset in regards to fasting?

 

According to the majority of scholars, a person must make up his fast if he eats after dawn assuming that dawn has not entered, or eats before sunset thinking the sun has set and finds out later that he was mistaken. Presumptions that prove to be erroneous are of no legal significance.
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 Suhaib al-Khayr [En. the generous] in Ramadan on a drizzling cloudy day. Suddenly the sun appeared, so Suhaib said: 'God fed us. Complete your fast until sunset and make up a day in its place.'"
God Almighty knows best.