The 24-Hour Clock

Imagine you are meeting a friend "at 8". Do you mean 8 in the morning or 8 in the evening? On an ordinary clock face you can't tell — the hand points to 8 twice a day. That is exactly the muddle the 24-hour clock sweeps away.

Train timetables, digital clocks, hospitals, the military and most of the world use the 24-hour clock because it is completely unambiguous. There is no am and no pm to mix up: 08{:}00 can only be the morning, and 20{:}00 can only be the evening. One time, one meaning.

How the system works

On a 12-hour clock the hours run 1 to 12 twice a day, so we tag each one am (morning) or pm (afternoon and evening). The 24-hour clock just keeps counting straight through: after 12{:}00 (noon) you don't start again at 1 — you go on to 13, 14, all the way to 23, then round to 00{:}00 at midnight.

Times are written as four digits — two for the hour, two for the minutes — like 09{:}00, 14{:}30 and 21{:}15. To turn an afternoon or evening (pm) time into 24-hour time, you simply add 12 to the hour: 1\text{ pm} = 13{:}00, 2\text{ pm} = 14{:}00, …, 11\text{ pm} = 23{:}00.

12-hour 24-hour
12:00 am (midnight)00:00
7:00 am07:00
12:00 pm (noon)12:00
1:00 pm13:00
3:00 pm15:00
9:00 pm21:00
11:00 pm23:00

See the whole day on one line

Picture a day stretched out as a straight line. Midnight is at the far left (00{:}00), noon is in the middle (12{:}00), and the next midnight is at the far right (24{:}00, which is the same as 00{:}00). Every pm time lives in the right-hand half — exactly 12 further along than its am twin.

Worked example 1 — turn pm into 24-hour

Convert 3:45 pm. It is a pm time, so add 12 to the hour: 3 + 12 = 15. The minutes never change. So 3{:}45\text{ pm} = 15{:}45.

Try 8:20 pm: 8 + 12 = 20, so 20{:}20. And 7 am is a morning time, so the hour stays the same — but we write it with a leading zero to make four digits: 07{:}00.

Worked example 2 — turn 24-hour back into am/pm

Convert 20:15. The hour 20 is bigger than 12, so it is a pm time. Subtract 12 from the hour: 20 - 12 = 8. So 20{:}15 = 8{:}15\text{ pm}.

What about 09:30? The hour is smaller than 12, so it stays a morning time: 9{:}30\text{ am}. And 15{:}00? 15 - 12 = 3, so 3\text{ pm}.

Worked example 3 — how long until the train?

The clock says 13{:}20 and your train leaves at 16{:}50. Because both times are in 24-hour form, there is no am/pm to trip over — just count on. From 13{:}20 to 16{:}20 is 3 hours, then 16{:}20 to 16{:}50 is 30 minutes. So the wait is 3 hours 30 minutes.

This is where the 24-hour clock really shines: working out durations across noon (11{:}00 to 14{:}00 is just 3 hours) never confuses you the way "11 am to 2 pm" can.

Two "12" times are the classic traps. Adding 12 works for every pm hour except these two:

Everything else follows the simple rules: a pm hour adds 12 to the hours only (never the minutes), and a morning (am) time keeps its hour but gains a leading zero (7\text{ am} = 07{:}00) — with midnight, 12\text{ am} = 00{:}00, as the one exception.

"Let's meet at 12" is genuinely dangerous — does it mean noon or midnight? Countless missed flights, late deliveries and mixed-up appointments trace straight back to am/pm confusion. A single misread 7{:}00 can send an ambulance, a train or a pilot to the wrong hour of the day.

That is exactly why hospitals, airlines, the military and railways all use the unambiguous 24-hour clock, where every moment has one and only one name. In fact most of the world uses it by default — it is mainly a few English-speaking countries that still say "half past seven in the evening" instead of the crisp, unmistakable 19{:}30.

See it explained