Becoming Eligible to Fast During the Day

Q: If someone embraces Islām during the daylight hours of Ramadān, are they obliged to fast the remainder of the day?

A: If someone were to embrace Islam, regain mental faculty, or conclude their menstrual cycle or postpartum bleeding during the daylight hours they are obliged to fast the remainder of the day and then make it up later.

Source: Sharh al-Muntaha