I've come across this nice picture by Leo Reynolds that may come in handy to revise how to tell the time in English.
You may see that not all the times are sharp, but to overcome this difficulty we can use the prepositions before and after:
It's just before two o'clock.
It's (two minutes) after a quarter past five.
Self-study activity:
Get together with an English-speaking friend or just on your own and tell the time that the different clocks show.