The units of time
Time is the one magnitude where the metric system never took hold. The second is the SI base unit, defined since 1967 by the caesium atom, but everything above it is sexagesimal and inherited from Babylonian astronomy: sixty seconds to the minute, sixty minutes to the hour, twenty-four hours to the day. Attempts at decimal time, including a serious French one in the 1790s, all failed.
| Unit | Symbol | Equals |
|---|---|---|
| Milliseconds | ms | 0.001 s |
| Seconds | s | 1 s |
| Minutes | min | 60 s |
| Hours | h | 3,600 s |
| Days | d | 86,400 s |
| Weeks | wk | 604,800 s |
| Years | yr | 31,557,600 s |
How to convert by hand
Every time conversion follows the same procedure: take the amount to the base unit, then from there to the target unit. In practice it is a single multiplication, because the two steps combine into one factor.
Convert to seconds and back out. The factors are 1,000 for milliseconds, 60 for minutes, 3,600 for hours and 86,400 for days. A year here is 365.25 days, which averages in the leap years.
The equivalents people look up most
| Conversion | Result |
|---|---|
| An hour | 60 min · 3,600 s |
| A day | 24 h · 86,400 s |
| A week | 7 d · 604,800 s |
| A year | 365.25 d · 31,557,600 s |
| A blink | 300 ms · 0.3 s |
Common mistakes
- Treating 1.5 hours as 1 hour 50 minutes. Decimal hours are not hours and minutes: 1.5 h is 1 h 30 min.
- Assuming every year is 365 days. Over any stretch longer than a few years the leap days matter.
- Assuming every day has 86,400 seconds. Daylight saving changes make two days a year 23 or 25 hours long in most of Europe and North America.
- Adding times as if they were decimals. 45 minutes plus 30 minutes is 1 h 15 min, not 75 minutes expressed as 1.75 h.