It doesn't matter how you write it so long as you write all other times in the same way.
Unless it's speech, then you just write it however your character says it.
Like in the text you might write times as: 6:45am, or six forty-five am, yet in speech a character may say, "a quarter to seven", all that's fine, what you don't do is write it as "1/4 to seven" which is a mixture of 2 conventions.
Then when your MSS is accepted for publication an editor my ask for all times to be in figures no matter what - well, they are paying you, so as long as it's not distorting a clue, I'd say okay.