I was wondering about this the other day while preparing a proof and I thought I'd look it up to be sure. I'm really glad that I did look it up, because I was hyphenating when I shouldn't have been. *sigh* It seems I'm always over-punctuating...
Here are some helpful rules on how to write time from this website (which, incidentally has lots of other handy grammatical rules).
Rule 12Normally, spell out the time of day in text even with half and quarter hours. With o'clock, the number is always spelled out.
She gets up at four thirty before the baby wakes up.
The baby wakes up at five o'clock in the morning.
Rule 13Use numerals with the time of day when exact times are being emphasized or when using A.M. or P.M.
Monib's flight leaves at 6:22 A.M.
Please arrive by 12:30 sharp.
She had a 7:00 P.M. deadline.
Rule 14Use noon and midnight rather than 12:00 P.M. and 12:00 A.M.
Rule 15Hyphenate all compound numbers from twenty-one through ninety-nine.
Forty-three persons were injured in the train wreck.
Twenty-three of them were hospitalized.