"until 20.00 hours". How to pronounce "20.00 hours" ?
I would say "twenty hundred hours". I would also remove the full stop from the written version (2000 hours). I used to work at an airport where everything had to be done using the 24-hour clock so I am used to using this terminology.
The flight's due in at sixteen thirty-four. (ETA 1634)
It took off at nineteen hundred hours. (ToD 1900)
It's due to take off at ten twenty-seven. (ETD 1027)
(ETA = Estimated Time of Arrival)
(ToD = Time of Departure)
(ETD = Estimated Time of Departure)