Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) says concerning a person who owes the making up of the month of Ramadan that he should make it up separately, and Allah Almighty says: "Then a number of other days" [Al-Baqarah: 184], that is, complete that number on other days.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8239