We measure time with a handful of fixed facts. Each bigger unit is just a bundle of smaller ones:

• 60 seconds = 1 minute;
• 60 minutes = 1 hour;
• 24 hours = 1 day;
• 7 days = 1 week;
• 12 months = 1 year;
• 365 days = 1 year (366 in a leap year).

The months are not all the same length: most have 30 or 31 days, while February has 28 days — or 29 in a leap year.

A duration is how long something lasts. To find it, you count on from the start time to the end time. It is often easier to count up to the next whole hour first, then carry on from there.

For example, a film that starts at 2:00 pm and ends at 4:30 pm: count 2 whole hours from 2:00 to 4:00, then 30 more minutes to 4:30. That is 2 hours and 30 minutes — or 2 \times 60 + 30 = 150 minutes in all.