która godzina? what's the time?

 to state the time, we say:

jest godzina...

or

jest...    followed by an ordinal number

so we're saying it's the first / second / third hour

if we are not using the 24 hour clock, we may need to say that it's

in the morning     -    rano

just before noon    -    przed południem        e.g: for 11am

in the afternoon    -    po południu

in the evening    -    wieczorem

at night    -    w nocy

and if we are using the 24 hour clock we carry on with the ordinal numbers up to midnight, północ,  which in Polish is stated as the 24th hour as opposed to our zero hour.

When saying something is at a certain time we say: ...o + the time in the genitive case

essentially replacing the a at the end of the ordinal number with ej for all numbers except druga where you replace the a with iej.

Times before and past the hour (using the 12 hour clock)

When stating the time, the minutes to the hour are stated as a normal cardinal number (one, two, three, etc.) and the hour is stated as an ordinal number (ending -a).

to the hour    -    za

np. za dziesięć druga    -    ten to two

When stating the time, the minutes past the hour are stated as a normal cardinal number (one, two, three, etc.) and the hour is stated as an ordinal number, but in the genitive case (ending -ej).

past the hour    -    po

np. dziesięć po dziesiątej    -    ten past ten

Re. word order: handily the za seems to go before the hour and the po goes after

Half past is half to the next hour

08:30    -    jest w pół do dziewiątej

at 08:30    -    o wpół do dziewiątej

Times using the 24 hour clock

Here we only state minutes past the hour.

With the hour as an ordinal number followed by the minutes as cardinal numbers

09:40    jest dziewiąta czterdzieści

15:30    jest piętnasta trzydzieści

19:15    jest dziewiętnasta piętnaście

as before, saying at a certain time starts with o and the hour is in the genitive case (-ej)

...at 15:30   -  ...o piętnastej trzydzieści