Units of Time: Full Conversion Table and Examples

Units of time express how long something lasts, from seconds up to centuries. Unlike units of length and units of mass, which use a ladder of tens, time has no single pattern — and that is precisely why it goes wrong so often.

The Conversion Table

Longer Units of Time

This is the section asked most often in primary tests and forgotten most often:

In ascending order: lustrum, windu, decade, century, millennium. The numbers follow no pattern, so they do have to be memorised — but only these five lines.

Leap Years

A leap year has 366 days, with 29 days in February. The rule: a year divisible by 4 is a leap year, except century years, which must be divisible by 400.

• 2024 is divisible by 4 → leap year
• 1900 is divisible by 4 but not by 400 → not a leap year
• 2000 is divisible by 400 → leap year

Worked Conversions

2 hours 15 minutes = ? minutes → (2 × 60) + 15 = 135 minutes

3 windu + 2 lustrums = ? years → (3 × 8) + (2 × 5) = 24 + 10 = 34 years

1.5 hours = ? seconds → 1.5 × 3,600 = 5,400 seconds

Finding a Time Difference

An event starts at 08:45 and ends at 11:20. How long does it last?

Subtract by borrowing 60, not 10:

• 11:20 − 08:45
• Minutes: 20 − 45 will not do, so borrow 1 hour = 60 minutes → 80 − 45 = 35 minutes
• Hours: 10 − 8 = 2 hours
• The event lasts 2 hours 35 minutes

Borrowing 60 rather than 10 is the only thing that separates time subtraction from ordinary subtraction.

The Mistakes That Actually Cost Marks

1. Borrowing 10 when subtracting times. Children are trained on the decimal system and borrow 10 automatically. Time works in base 60. This is the number one cause.

2. Writing 1 hour 30 minutes as 1.30. Its value is 1.5 hours. The same error wrecks every speed question, so fixing it here fixes two chapters at once.

3. Mixing up windu and decade. A windu is 8 years, a decade 10. There is no logic to it, only memory.

4. Treating every month as 30 days when working with real dates. Rounding is fine for conversion drills, but actual date calculations need the real day counts.

5. Assuming every century year is a leap year. 1900 was not; 2000 was.