Here in Germany there are usually different mehtods of telling the time, depending on the clock face and how precise it needs to be.
When I'm reading an analog clock, I typically use a 12-hour system and because it usually doesn't need to be precise I round to the next quarter or tenth. Some examples:
15:45: "Dreiviertel Vier" (three quarters four)
15:15: "Viertel Vier" (quarter four)
15:20: "Zehn vor halb Vier" (ten before half four)
I think the "Dreiviertel" and "Viertel" thing is specific to this part of Germany, most people use "Viertel vor X" (quarter to X) and "Viertel nach X" (quarter past X).
When I read the digital clock, I usually use a 24-hour system and also state the minute:
15:34: "Fünfzehn Uhr Vierunddreißig" (fifteen [clock] thirty-four)
On an analog clock for me this would just be "Halb Vier" (half four).
When I write the time I usually just leave the ":" out (like "1534") but most people here keep it.
Do you use similar systems in your language? And what are the differences?