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The answer is not always so simple. The Czech case system has lots of rules when it comes to numbers, so the answer to "what time is it?", isn't an easy one.
The Czech word for hour or "o'clock" is hodina. It is 1 o'clock is Je jedna hodina.
Hodina is a feminine noun and the numbers 2, 3 and 4 require the nominative plural form. This changes hodina to hodiny. It is 2 o'clock is Jsou dvě hodiny. (Literally...they are 2 hours)
Hang on now because that was the easy part. Things get trickier when it comes to quarter and half hours. In English, we think of the current hour. In Czech, you think of the upcoming hour.
In English, 5:45 is "a quarter to six". But in Czech it is třičtvrtě na šest (three quarters towards six).
In American English, 5:30 is "five thirty" or "half past five".
In British English, it is "half five".
In Czech, it is půl šesté, (half six - half "towards" six). Another tricky point...when you use půl, the following number gets treated like an adjective so šest (6) becomes šesté (6-ish).
The correct way to say 5:25 is za pět minut půl šesté (in five minutes half towards six).
1:51 is třičtvrtě na dvě a šest minut (three quarters towards two and six minutes). It's like a secret code or something.
Czechs use the 24-hour clock. I've noticed that things get simpler after 13:00 (1 pm). People seem to just read the numbers off. I've never heard anyone say "half towards 18:00" for 5:30 pm. One would simply say "half towards 6".
I swear that calculus was easier to learn than telling time in Czech. It is rather humbling to get one's butt kicked by a clock. Sometimes I think it would be easier to just wear a digital watch so that I could just hold up my watch when I get asked what time it is. And as long as my watch is running, or if my mobile is handy, then I don't ever have to ask what time it is anyway.