Spell out the time of day when it is on a quarter hour or half hour.
He gets up at six thirty every morning.

Spell out the time when using o’clock.
The meeting is at seven o’clock.

Use numerals when an exact time is being emphasized.
The flight leaves at 8:23.

Use numerals when using a.m. or p.m.
The ceremony will be at 7:00 p.m. on Thursday.

Use noon and midnight instead of 12:00 a.m. or 12:00 p.m.
The luncheon will be at noon on Friday.