From Citizendium, the Citizens' Compendium
- Time : A fundamental quantity in physics - that what differs always between two inspections of the same system by the same observer.
- Calendar : A method used for keeping time on a scale of years.
- Week : Time unit equal to a number of days, now usually seven calendar days, previously weeks of between 4 and 20 days have been used historically in various places.
- Day : Defined as 86,400 seconds, using the SI definition of second.