Answered by Shaykh Faraz Rabbani
Question: Up until what time can one make an intention to fast?
Answer: The intention in fasting can be made anytime until the Islamic mid-day, which is half-way between the entry time of Fajr and Maghrib. This is roughly around an hour before Zuhr. An intention may also be made any time from Maghrib of the night before, and remains valid as long as you did not resolve not to fast.
Note, however, that an “intention” is “a firm resolve to perform an act of worship.” In relation to fasting, “it is to know in your heart that you will fast that day.” [al-Fatawa al-Hindiyya, quoting al-Khulasa] As such, it is rarely conceivable that a Muslim not have an intention in Ramadan, the scholars tell us. It is even explained that the very intention to get up for the pre-dawn meal (suhur) is an effective intention to fast. [Mentioned by Abu Bakr al-Haddadi in his Jawhara al-Nayyira, quoting Najm al-Din al-Nasafi, and in al-Fatawa al-Hindiyya; and by Ibn Abidin in Radd al-Muhtar, quoting al-Dhahiriyya via Ibn Nujaym’s al-Bahr al-Raiq]
And Allah knows best.