Number that is used to express a duration (duration being a space of time that elapses in relationship to a phenomenon) between two observed or observable limits, which are the start and end of the phenomenon.

### Examples

- Marianne played ball for two hours.
- Julianne skated for 30 minutes.
- The film lasted 2 hours and 20 minutes.

### Notations

To express a duration, we use these conventional units of measure, among others:

- The second (s) is the base unit.
- One minute (min) is equal to 60 seconds.
- One hour (h) is equal to 60 minutes.
- One day (d) is equal to 24 hours.
- One year (y) is equal to 365 days.
- One decade is equal to 10 years.
- One century is equal to 100 years.
- One millenium is equal to 1000 years.

Weeks, months, and seasons are not conventional units. They do not have specific symbols.

- 1 week is equal to 7 days
- 1 month is equal to 28, 29, 30 or 31 days
- 1 year is equal to 12 months
- 1 season is equal to 3 months

There are 12 months in a year.