Unit 8 · Topic 02 · Money and Time
The train ticket said the train left at 18:45.
Kabir read it as "6:45" and nearly missed it — he forgot to check which 6:45.
One clock, two ways of writing it, and the difference genuinely matters.
Ms. Rao held up a real train ticket: Departure 18:45. "Kabir, what time does this train leave?"
Kabir looked at the clock on the wall — a normal 12-hour clock. "There's no 18 on this clock. It only goes up to 12."
"That's because this ticket uses the 24-hour clock," said Ms. Rao. "Instead of restarting at 1 after 12 noon, it just keeps counting: 13, 14, 15... all the way to 23, then back to 00 at midnight. It never needs 'am' or 'bm' labels, because every hour of the day has its own unique number."
"So how do I turn 18:45 into a normal clock time?" asked Kabir.
"For any 24-hour time above 12, subtract 12 to get the 12-hour version, and it's always the afternoon or evening." Kabir subtracted: 18 − 12 = 6. "So 18:45 is 6:45 in the evening — 6:45 pm."
"Exactly. And for times from 00:00 to 12:00, the 24-hour clock and the 12-hour clock mostly agree — 09:15 is just 9:15 am, no subtraction needed. The only two special cases are 00:00, which means 12 midnight, and 12:00, which means 12 noon."
Anaya tried the reverse direction: "So if a normal clock says 3:20 pm, what's that in 24-hour time?"
"Since it's pm and not 12 itself, add 12," said Ms. Rao. 3 + 12 = 15. "15:20."
"And 3:20 am?" "Am hours before noon stay exactly the same in 24-hour time — 03:20, just written with a leading zero."
Ms. Rao then set the real test: a bus timetable listing 07:05, 13:40, and 21:15. Kabir converted each: 07:05 stays 7:05 am (no change needed). 13:40 becomes 1:40 pm (13−12=1). 21:15 becomes 9:15 pm (21−12=9).
"Missing a train because you read the ticket as morning instead of evening is exactly the kind of mistake this method prevents," said Ms. Rao. "Always check: is the 24-hour number above 12? Then it's afternoon or evening, and you subtract."
The 24-hour clock numbers every hour of the day uniquely from 00 to 23, so it never needs 'am' or 'pm' labels — 00:00 is midnight, 12:00 is noon, 23:59 is one minute before the next midnight.
To convert a 24-hour time above 12 into 12-hour time: subtract 12, and the result is pm (afternoon/evening). Example: 18:45 − 12 = 6:45 pm.
To convert a 24-hour time of 12 or below into 12-hour time: it stays mostly the same, except 00:00 becomes 12:00 midnight and 12:00 stays 12:00 noon.
To convert 12-hour pm time (except 12 itself) into 24-hour time: add 12. To convert 12-hour am time into 24-hour time: it stays the same, just written with two digits.
Find 5 times written in 24-hour format (a train or bus timetable, a digital clock) and convert each to 12-hour time.
Write your own daily schedule (wake up, school, dinner, bedtime) in both 12-hour and 24-hour format.
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