Definition
12-hour time
12-hour time uses the hours 1 to 12 twice each day, with a.m. and p.m. showing which half of the day is meant.
What it means
The clock numbers repeat after every 12 hours. That means 7:20 happens once in the morning and once in the evening. Add a.m. or p.m. so the reader knows which one you mean. At exactly 12:00, the words noon and midnight are clearest.
Remember it
Same twelve clock numbers, two halves of the day—add a.m. or p.m.
Example
A film starts at 7:20 p.m. The p.m. label tells you it starts in the evening, not at 7:20 in the morning.
Watch for
Writing 7:20 without a.m. or p.m. does not identify morning or evening unless the surrounding context makes it clear.
Common mix-up
The 12 comes before the 1 in each cycle: 12:30 a.m. is after midnight, while 12:30 p.m. is after noon.
Exact meaning
A 12-hour clock labels the hours 1 through 12 in two cycles each day. The a.m. label is used from midnight until before noon, and p.m. from noon until before midnight; saying noon or midnight at exactly 12:00 avoids ambiguity.