question #1 Sahih Muslim 1083, Book 13, Hadith 26 : How does the lunar calendar affect the determination of the length of a month in Islamic tradition?

When twenty-nine nights were over, which I had counted, the Messenger of Allah (ﷺ) came to me (he came to me first of all). I said: Messenger of Allah, you had taken an oath that you would not come to us for a month, whereas you have come after twenty nine days which I have counted. Whereupon he said: The month may also consist of twenty-nine days.