The rule depends on two independent questions: is the fast already assigned to a date, and is the intended duty identifiable? The date helps determine Ramadan and a specified vow. An unspecified outstanding duty has no such automatic marker.
Hence a valid intention for an outstanding obligation must identify the fasting duty, and the earlier timing requirement applies.
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Kitab al-Sawm: Categories of Fasting and Rules of Intention
The rule depends on two independent questions: is the fast already assigned to a date, and is the intended duty identifiable? The date helps determine Ramadan and a specified vow. An unspecified outstanding duty has no such automatic marker.
Hence a valid intention for an outstanding obligation must identify the fasting duty, and the earlier timing requirement applies.