Midnight and Midday Convention
The universal convention is that 12:00 AM signifies midnight, and every moment afterward is also AM until the moment of 12:00 PM which signifies midday.
Midnight Etymology and Morning-Night Conundrum
In ancient Roman timekeeping, midnight was halfway between sunset and sunrise, varying according to the seasons. The dark hours after 12 midnight can colloquially be referred to as either morning or night.
Time Description and Latin Origin
An example: 10.00 a.m. is 10 o-clock in the morning. PM stands in short for Post Meridiem, the Latin name for “After Midday” or “After Noon”. An example: 10.00 p.m. is 10 o’clock in the evening.