Time — Clock & Calendar • Topic 2 of 6

Time Formats

There are two ways to write time, and CTET likes to test the conversion between them. The 12-hour clock runs the numbers 1 to 12 twice a day, so it needs the labels a.m. and p.m. to say which half you mean. A.m. (ante meridiem, 'before midday') covers midnight through to just before noon; p.m. (post meridiem, 'after midday') covers noon through to just before midnight. The two values everyone confuses are the twelves: 12:00 midnight is 12 a.m. and begins the day, while 12:00 noon is 12 p.m. and sits in the middle of it. The 24-hour clock — railway or 'military' time — sidesteps the labels entirely by numbering the hours 0000 to 2359. To convert, leave morning hours as they are (1:00 a.m. is 0100), but turn 12 a.m. into 0000; for the afternoon and evening, add 12 to the hour (1:00 p.m. becomes 1300, 11:59 p.m. becomes 2359), with the one exception that 12 noon stays 1200. A frequent slip is converting 12:30 p.m. to 0030 instead of 1230 — only the hours after 1 p.m. take the +12. Tying these formats to a child's day (wake at 7 a.m., lunch at 1 p.m., bed at 9:30 p.m.) is the pedagogy point worth remembering.

✅ Solved examples

1. Convert 4:45 p.m. to the 24-hour format.
For p.m. times after 12 noon, add 12 to the hour: 4 + 12 = 16. So 4:45 p.m. = 1645 hours.
2. Write 0000 hours and 1200 hours in 12-hour format with the correct label.
0000 hours is 12:00 midnight = 12:00 a.m. (start of the day). 1200 hours is 12:00 noon = 12:00 p.m. (middle of the day).
3. A train timetable shows departure at 2110 hours. What is this in 12-hour time?
Subtract 12 from the hour for evening 24-hour times: 21 - 12 = 9. So 2110 hours = 9:10 p.m.
4. A student converts 12:30 p.m. to 0030 hours. Is this correct? Explain.
No. The +12 rule applies only from 1 p.m. onward; 12 noon stays as 1200 hours. So 12:30 p.m. = 1230 hours, not 0030 (which would be 12:30 a.m., after midnight).

✏️ Practice — try these, take hints as needed

1. Convert 7:00 a.m. to the 24-hour format.
Morning hours keep the same number.
Pad to four digits.
0700 hours
2. Which is the start of a new day: 12:00 a.m. or 12:00 p.m.?
a.m. begins at midnight.
p.m. begins at noon.
12:00 a.m. (midnight)
3. Express 1830 hours in 12-hour time with a.m./p.m.
It is an evening time.
Subtract 12 from the hour.
6:30 p.m.
4. In the 24-hour clock, how is 11:59 p.m. written?
Add 12 to the hour 11.
It is the last minute of the day.
2359 hours

📝 Topic test — 8 questions

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