When does the intention for a specific, rather than an absolute, voluntary fast begin?

For absolute voluntary fasting (Nafilah Mutlaqah), it is not a condition to form the intention (niyyah) from the night before; rather, it is sufficient to fast with an intention made during the day, provided no nullifier of the fast has been committed prior to the intention. As for specified voluntary fasting (Nafilah Mu'ayyanah), it is a condition to form the intention from the night before. The reward for one who fasts with an intention made during the day is only from the time of the intention, not for the entire day. Consequently, whoever intends to fast a specific voluntary fast, such as "the six days of Shawwal" or "the Day of 'Arafah," during the daytime, will not receive the full reward for fasting the entire day, which is contingent upon fasting the complete day.