Say time in 24 hour format I've never seen in Chinese study materials that time could be said/written in 24 hour format. It is always in 12 hour format with addition of description like 早上, 下午... E.g.:


早上八点半
晚上九点四十五分


And never like:


二十一点四十五分


Is it ok to say/write time in 24 hour format?
It's no problem to use either in practice, though people used 12 hour format more (because 6 is easier to say than 18 ). When being asked: 现在几点, I might answer either 6点半 or 18点30. 18点30 sounds a little bit formal or "verbose".