The gigasecond (Gs) is a unit of time in the International System of Units, defined as 109 seconds using the SI prefix system. Prefixed SI units of time greater than a second are rarely used because the traditional units for periods longer than a second are still very widespread. One gigasecond is approximately 31.688 years. People reach one gigasecond old 31 years and 251 days after they are born, which can be considered a milestone. They could say "I am one billion seconds old, we should throw a gigasecond party!" They turn two gigaseconds old at 63 years and 137 days of age, which is around the age when they typically retire and become senior citizens. The average life expectancy of humans around the world is approximately 2.24 gigaseconds (71 years) and one century is 3.155 gigaseconds