If one vows to fast for a month, does he fast twenty-nine days or thirty?

Question : Shaykh, may Allah reward you. If one vows to fast for a month.

Answer : Yes. He fasts twenty-nine or thirty days. If he intended a specific month, it is as it was. If he said "a month" and entered it from the first day of the month, intending to complete that month, then it is the number of days in that month, whether twenty-nine or thirty. As for if he intended a month and entered it from the middle of the month, he completes thirty days.