What should one who was unable to make up missed fasting days before the advent of the next Ramadan do?

The missed fasts of Ramadan must be made up immediately. Delaying the make-up without a valid excuse until the next Ramadan enters necessitates sin and expiation (a mudd of food for each day). However, delaying due to an excuse, such as an illness from which recovery is hoped, only requires making up the fast, without feeding. As for chronic illness or old age that prevents fasting, it only requires feeding (food for one needy person for each day).