That is, at the precise stroke of 12:00:00. That time, along with 12:00:00 noon, are technically neither AM or PM because AM and PM mean "ante-meridiem" and "post-meridiem", and noon and midnight are neither ante- nor post- meridiem. However, for convenience, most people lump the 12:00:00 time with its nearest neighbour, 12:00:01, which IS AM ... More @Wikipedia
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