The terasecond (Ts) is a unit of time in the International System of Units, defined as 1012 seconds using the SI prefix system. Positive multiples of seconds such as teraseconds are rarely used because the traditional units for periods longer than a second such as years and millennia are still almost always used. One terasecond is 31,688 years, meaning this unit can be used as an alternative of tens or hundreds of thousands of years. An example of this usage is that modern humans have been around for the last six teraseconds (approximately 190,000 years).