Time
We use time to sequence events, to compare their durations and the intervals between them, and to quantify the speed at which objects move and things change.
To measure time, we can use anything that repeats itself regularly. One example is the dawn of a new day (as Earth rotates on its axis). Two more are the phases of the moon (as it orbits the Earth), and the seasons of the year (as the Earth orbits the Sun). Even in ancient times, people developed calendars to keep track of the number of days in a year. They also developed sundials that used the moving shadows cast by the sun through the day to measure times smaller than a day. Today, highly accurate clocks can measure times less than a billionth of a second. The study of time measurement is horology.
The SI (Systeme Internationale) unit of time is one second, written as s.
In Einsteinian physics, time and space can be combined into a single concept. See spacetime continuum.
Units of time
 1 millennium = 1000 years
 1 century = 100 years
 1 decade = 10 years
 1 lustrum = 5 years
 1 year = 12 months ≈ 365 days (366 days in leap years)
 1 month ≈ 28 to 31 days ≈ 4 weeks
 1 fortnight = 14 days = 2 weeks
 1 week = 7 days
 1 day = 24 hours
 1 hour = 60 minutes
 1 minute = 60 seconds
 1 second = SI base unit of time
 1 millisecond = 1/1,000 second
 1 microsecond = 1/1,000,000 second
 1 nanosecond = 1/1,000,000,000 second
 1 picosecond = 1/1,000,000,000,000 of a second
 1 femtosecond = 1/1,000,000,000,000,000 of a second
 1 attosecond = 1/1,000,000,000,000,000,000 of a second
 1 Planck time = smallest measurable unit of time
Things to measure time
Time of day
Images

Horizontal sundial in Taganrog

Chipscale atomic clocks, such as this one unveiled in 2004, are expected to greatly improve GPS location.

Views of spacetime along the world line of a rapidly accelerating observer in a relativistic universe. The events ("dots") that pass the two diagonal lines in the bottom half of the image (the past light cone of the observer in the origin) are the events visible to the observer.

Philosopher and psychologist William James