Suppose someone made no decision the previous night, but abstained from invalidating actions since dawn. They later decide to observe a voluntary fast before the legal-day midpoint.
Under the stated conditions the fast may be valid as voluntary. It cannot be redesignated as a missed Ramadan or expiatory fast, each of which requires an overnight intention.
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Kitab al-Sawm: Categories of Fasting and Rules of Intention
Suppose someone made no decision the previous night, but abstained from invalidating actions since dawn. They later decide to observe a voluntary fast before the legal-day midpoint.
Under the stated conditions the fast may be valid as voluntary. It cannot be redesignated as a missed Ramadan or expiatory fast, each of which requires an overnight intention.