Rule: Spell out the time of day in text even with half and quarter hours.
Example: It was almost half past six when she got the first call from the press.
Rule: With o’clock, the number is always spelled out.
Example: The show begins at seven o’clock in the evening.
Rule: Use numerals to express the time of day when exact times are being emphasized.
Example: Her flight leaves at 6:22 a.m.
Rule: Use noon and midnight rather than 12 p.m. or 12 a.m.
Example: He was born at midnight.
The session will be adjourned by noon.
Note: Most style guides recommend a.m. and p.m. (lower case, with period; AM/PM may appear at times but is not the norm.)
Rule: Omit zeroes when the time is on the hour (7 a.m., NOT 7:00 a.m.) but include them in a list of varying times like this one:
9:00 a.m. Opening speech
9:30 a.m. Presentation by the Sales Team
10:00 a.m. Break