When is it pass midnight?

No such time as neither '12 pm'. nor '12 am'

It is either '12 noon' or '12 midnight'.

'am/pm' is the initials of the Latin phrase ' ante/post meridian ' ; meaning 'before/after noon'.

So if somewrites '12 am' , they are confusing you as to noon or midnight. Because if it is 12 noon , how can it be before noon or after noon. Similarly for midnight.