True Solar Time is one of the complications commonly used in astronomical watches. Our daily timekeeping is mean solar time, which is based on the assumption that the earth's orbit around the sun is a standard circle, and the time of day is uniform throughout the year. In fact, the earth's orbit around the sun is elliptical, and every day is not always 24 hours. The clock that sets the time based on this theory is called "true solar time". True solar time is equal to the sum of the mean solar time and the true mean solar time difference.

The 24-hour time system used on most wristwatches to time the day is "mean solar time", or more commonly known as "civilian time", a simple timekeeping system that ignores changes in the equation of time.