Convert from a larger unit to a smaller one by multiplying, and from a smaller unit to a larger one by dividing — exactly like metric conversions.

• Convert 3 hours to minutes: $3 \times 60 = 180$ minutes.
• Convert 120 minutes to hours: $120 \div 60 = 2$ hours.

The 12-hour clock uses a.m. (before midday) and p.m. (after midday), cycling 1–12 twice a day. The 24-hour clock runs straight from 00:00 to 23:59 with no a.m./p.m. labels.

• 12-hour → 24-hour: a.m. hours stay the same (9:30 a.m. → 09:30), except 12 a.m. (midnight) which becomes 00:00. For p.m. hours, add 12 (3:45 p.m. → 15:45), except 12 p.m. (midday) which stays as 12:00.
• 24-hour → 12-hour: hours 01–11 are a.m. unchanged; 00:00 is 12:00 a.m.; 12:00 stays as 12:00 p.m.; hours 13–23 have 12 subtracted and become p.m. (21:50 → 9:50 p.m.).

Break the journey into manageable chunks rather than subtracting directly, since time doesn't work in base 10. Go from the start time up to the next whole hour, then count whole hours, then add on the final partial hour.

Example: Find the time from 10:45 a.m. to 3:20 p.m.

1. 10:45 a.m. to 11:00 a.m. is $15$ minutes.
2. 11:00 a.m. to 3:00 p.m. is $4$ hours.
3. 3:00 p.m. to 3:20 p.m. is $20$ minutes.
4. Total: $15 \text{ min} + 4 \text{ h} + 20 \text{ min} = 4$ hours $35$ minutes.

A train departs at 21:40 and arrives at its destination 2 hours 55 minutes later. What time does it arrive, in 24-hour clock format, and what is this in the 12-hour clock?

1. Add the whole hours first: $21:40 + 2\text{ h} = 23:40$.
2. Add the remaining 55 minutes: $23:40 + 55 \text{ min}$. Since $40 + 55 = 95$ minutes, that's $1$ hour $35$ minutes past $23:40$.
3. $23:40 + 1\text{ h} = 24:40$, which rolls over past midnight to $00:40$ the next day.
4. In the 24-hour clock, the arrival time is 00:40. Converting to 12-hour format: since the hour is $00$, this is 12:40 a.m.