There are three separate English rules on how time values should be expressed. These rules depend upon how the number is said:
with “o’clock” …I got there at — o’clock.
with “A.M. or P.M.” …I got there at — P.M.
or by itself …I got there at —.
In our current era of searchable documents, I believe that we have to alter how times on the hour should be expressed so that the attorney can find them in his search. I believe the form should be with the colon and ciphers for all times on the hour.
The idea that “:00” means “o’clock” is one that grew up many, many years ago. I have never been able to confirm that as an actual rule, however. And in reporting, we do not, with rare exceptions, convert a word that is said into a symbol.
So if you are using the colon and ciphers for times on the hour, whether you are going to add “o’clock,” when it is said, truly depends on how verbatim you want to be. It is my opinion that, if “four” is said, “4:00” should be transcribed and that, if “four o’clock” is said, “4:00 o’clock” should be transcribed.
There are many who disagree with this.