If one has no intention to fast the next day during Ramadhaan, but about an hour after Fajr one decides to fast, will the fast be valid?

Answer

If it is the month of Ramadhaan, then the final time for the validity of the niyyat (intention) and for the Fast is the time called Nisfun Nahaar which is about one hour before Zawwaal. After Zawwaal begins Zuhr time.