الشرع إذا عين له وقتاً لا يجب غيره في ذلك الوقت.
When the law designates a measured period for a particular duty, competing obligations cannot ordinarily replace it during that very period.
A Ramadan day is legally assigned to the fast of Ramadan. A healthy resident who is obliged to fast cannot convert that day into a different obligatory fast merely by choosing another label.
Example: In the Hanafi analysis of the text, a resident's Ramadan fast intended as a make-up or expiatory fast is attributed to Ramadan because the period has already been legally specified.
This rule must not be uncritically extended to the distinct legal position of a sick person or traveler enjoying a dispensation.