Times of day in even, half, and quarter hours are spelled out in text. When including o’clock, the number is spelled out:

  • Her day begins at four o’clock in the morning.
  • The meeting continued until half past two.
  • We’ll start again at ten thirty.

Space before a.m. or p.m.; use full points in a.m. and p.m.; do not use a colon between hours and minutes.

The train leaves at 5.22 a.m. but you should be at the station by five o’clock.