Here is a real Irish Rail timetable line: a train leaves at 14:35. What time of day is that? Is it morning, afternoon, or night? Hands up if you can tell me what o'clock 14:35 means on an ordinary clock.

Each clock shows the same time two ways: the hands and the 24-hour digital readout. Watch how the hour changes once we pass midday. Which afternoon time surprises you most?

Today we work through these four times together on the clock, in this order: 15:00, 19:45, 22:10, and the night-time edge 00:30. For each one we will say it the 24-hour way, then the 12-hour way with am or pm.

Now we read times straight off the 24-hour clock and say them the ordinary way. For each one, set the hands, read the digital number, then tell us the 12-hour time with am or pm. Try these: 16:15, then 23:50, then the tricky 00:25 just after midnight, then 13:05.

What we learned today

• After midday, add 12 to the hours to write a 24-hour time, and subtract 12 to read it back as am or pm.
• Just after midnight the clock reads 00:something, never 24:something.
• Transport timetables use 24-hour time so a time can only mean one moment in the day.