I go to bed at nine fifteen pm - we don’t have to use pm here as it is obvious is is PM not AM I go to bed at nine fifteen (9:15) o'clock at night - o’clock means the whole hour so only to be used as 9 o’clock - you would say 9.15 or quarter past nine. - you would also still say in the evening, not night. I go to bed at twenty one fifteen o'clock - again can’t use o clock as it is not a whole hour - 24hr format is more formal. Not really used when you speak to your friends for example. It is to specify time of travel or appointment so there is no confusion between AM and PM I go to bed at quarter after nine o'clock at night - quarter past nine Remember Quarter past Half past Quarter to ( the nearest hour) And o ‘clock ( whole hour)

I go to bed at nine fifteen pm is the only natural sounding sentence. In American English, I have never heard o'clock used with partial hours. We say 9 o'clock, 10 o'clock, etc. But never 9:15 o'clock. The 24-hour clock is rarely used in the States. Unless a person has been in the military or lived outside the US, most people don't really understand.

You can say “at 9:15”, “at nine fifteen” or “a quarter past nine”.

#1 good #2 omit "o'clock" #3 omit "o'clock" #4 omit "o'clock" I would just say "I go to bed at nine fifteen" (or "quarter past nine"). It is not necessary to specify "pm" or "at night".