Solar time is the time determined from the Earth's orbit around the Sun. The definition of time comes from astronomy and refers to the Earth 's rotation and orbit around the Sun - one year.
Astronomical time varies by several milliseconds each day due to tides and weather effects. In addition, astronomical time lags over the years due to continental drift and the Earth's slowing rotation.
The time reference for astronomical time is Greenwich Mean Time (GMT), and that for technical time, Coordinated Universal Time (UTC). Both time scales differ by about 1 second per year, which is compensated by the insertion of the leap second.