Muslims should know that the majority of scholars have maintained the opinion that whoever deliberately neglects obligatory fasting for no excuse has to make up all the days he missed even if they happen to be years. No compensation is accepted in this regard. Of course, we do not know when are we going to die, but we are required to make sincere repentance and start to remedy the wrongs we fell in.
Hence, whoever has neglected fasting of some years in the past should try to make up for what he has missed. If he does not know the exact number of fast days he missed, he has to fast days that make him quite sure that he has made up for the days he missed of fasting.
