Qaḍāʾ means fasting a replacement day for a Ramadan obligation that was not validly fulfilled. One cannot return to that morning, so the replacement is observed on a different day.
One mistaken day entails make-up of that day; the passage does not invalidate all other correctly performed days of Ramadan.
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Kitab al-Sawm: Mistaken Dawn or Sunset, Qada and Kaffarah
Qaḍāʾ means fasting a replacement day for a Ramadan obligation that was not validly fulfilled. One cannot return to that morning, so the replacement is observed on a different day.
One mistaken day entails make-up of that day; the passage does not invalidate all other correctly performed days of Ramadan.