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General Data > Units of Time

The table below shows units and values for various periods of time. The SI base unit for time is the 'second'. Larger units such as 'hour' and 'day' are derived from the second.

Common Units of Time


Unit Size Notes
Femtosecond 10−15 second  
Picosecond 10−12 second  
Nanosecond 10−9 second  
Microsecond 10−6 second  
Millisecond 1/1,000 second  
Second SI base unit  
Minute 60 seconds  
Hour 60 minutes  
Day 24 hours  
Week, Sennight 7 days  
Fortnight 14 days; 2 weeks  
Month 28 to 31 days  
Quarter 3 months  
Year 12 months  
Tropical year 365.24219 days Average
Olympiad 4 years  
Lustrum 5 years obsolete
Decade 10 years  
Indict 15 years obsolete
Score 20 years  
Generation 25 years approximate
Century 100 years  
Millennium 1,000 years