That's easy, actually. The 12-hour clock is a colloquial version of telling time. If it isn't already clear from the context, you add du matin, de l'après-midi or du soir to the time to make sure whether you're talking about 6 am or 6 pm. Also, you don't usually specify time down to the last minute colloquially, rounding off to the nearest quarter hour or the nearest 5 minutes. Phrases such as moins le quart or et quart, et demi, etc. also belong to the 12-hour clock.

The appropriate lesson for this is here: Telling time in French - general 12-hour clock rules