Question: 12 am means next day?
Answer: 12:00am is midnight. 12:00pm is noon. Midnight is usually defined as 12:00:00 a.m., even though logically it's neither "ante-meridiem" nor "post-meridiem". In fact a new day is usually defined as beginning at 12 a.m. exactly, even though no time has elapsed in the new day at that time.