English: What time shall we meet tonight?
Czenglish: How about 19 o’clock?
English: Let’s go for 7 o’clock. You can only use o’clock from 1 to 12 (am or pm).
Czenglish: But what if I want to make sure you know it’s the evening?
English: Then you say 7pm.
We call the 24 hour clock ‘military time’ and generally speaking the military are the only ones who use it.