وَإِذَا بَلَغَ الصَّبِيُّ، أَوْ أَسْلَمَ الْكَافِرُ فِي بَعْضِ نَهَارِ رَمَضَانَ: أَمْسَكَا بَقِيَّةَ يَوْمِهِمَا، وَصَامَا مَا بَعْدَهُ، وَلَمْ يَقْضِيَا مَا مَضَى.
Translation: If a child reaches puberty, or a non-Muslim embraces Islam, during a Ramadan day, both abstain for the rest of that day, fast on subsequent days, and do not make up what preceded it.
The text describes two circumstances: a child reaches puberty during a Ramadan day, or a non-Muslim embraces Islam during a Ramadan day. Both must abstain for the rest of that day and fast on following days, while not making up the preceding period.
Abstention during the remaining hours is not the same as having performed a complete valid obligatory fast from dawn; the legal duty was not present at the start of that day.