When is it obligatory to make up a fast for someone who ate forgetfully after the dawn (Fajr) call to prayer?

The time for making up missed fasts is between the two Ramadans. It is not permissible to delay making up missed fasts from one Ramadan until the next Ramadan enters, except for a valid excuse. Whoever delays it without a valid excuse must feed a mudd (a measure of food) for each day.