Almost in all systems of unit second is the unit of time, most probably because, second is the most convenient one, if we look at the guidelines for selecting a unit.
For more details refer to the following links
- http://www.aticourses.com/international_system_units.htm
- http://www.french-metrology.com/en/history/history-mesurement.asp
- http://physics.nist.gov/cuu/Units/history.html
- http://en.wikipedia.org/wiki/Metric_system
- http://en.wikipedia.org/wiki/Systems_of_measurement
