How to say time correctly??

Do Brits say "I have dinner at 18 o'clock"
or "I have dinner at 18 in the evening".
And one more sentence "I study online from 9:15 to 14".

Help me, please 🙂
Thank you!

Question

How to say time correctly??

Do Brits say "I have dinner at 18 o'clock"
or "I have dinner at 18 in the evening".
And one more sentence "I study online from 9:15 to 14".

Help me, please 🙂
Thank you!

Answer

Us Brits don't speak in 24-hour format (except in the military). 
We say "I have dinner at 6", "I have dinner at 6 tonight", "I study from quarter past 9 till 2".

From the time of day and/or context we know when you mean A.M. or P.M.

Answer

Hey, Alina. I have a dinner at 6. I study from quarter past 9 to two ( for more clarity you can say from 9 in the morning OR from quarter past 9 to 2 pm). I hope it helps