If not taking into account the regional differences and the possibilities that are offered by the English language and just have a look at the system of rules it is quite clear and simple. We have the morning (a.m.) and the afternoon (p.m.) and the time is given by adding minutes (or parts of an hour) to the hour.

It's 10.15 a.m.
It's   5.50 p.m.

Of course then there are the possibilities to say

it is a quarter past 10 a.m.
it is ten minutes to five p.m.

Well, in any case between the whole hour and half past usually past is used to give the time.
Between half past the hour and the next full hour it is x minutes to the full hour.

The Spanish system is following more or less the same logic.