Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) held that making an exception is valid even if it comes a year later. Then he recited: "And never say of anything, 'I will do that tomorrow,' except (with the words), 'If Allah wills.' And remember your Lord when you forget" [Surah al-Kahf: 23-24], that is, whenever you remember.
Then Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said: "Whenever he remembers to make the exception, let him make it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / 19931