Additional tool: Definition: Time is a fundamental quantity which cannot be defined in terms of classical or newtonian quantities. The SI base unit of time is the second and is defined as the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom. This gives the unit second a stable unchanging reference value unlike references from the motion of the sun and planets.

Time can be measured either in "days of fixed length": SI days of 86400 SI seconds, defined by the atomic clocks, or dynamical days defined by the motion of the Moon and planets); or in "natural" days, defined by the rotation of the Earth with respect to the Sun. The duration of the natural day is steadily getting longer as measured by the SI second.

At the epoch J2000 (1 January 2000, 12h TT), the mean tropical year was: 365.242189670 SI days. Due to changes in the precession rate and in the orbit of the Earth, there exists a steady change in the length of the tropical year (about 5 ms/year). This means that in 2000 years time the tropical year will be about 10 seconds shorter.

The calculator uses the SI second, minute and hour. The week is based on a seven SI days. The month is based on 1/12 average calendar year.