What local time is it? To calculate local sun (TT) time, use the following simple formula: TT = GMT + LC + EOT, where GMT - mean sun time on 0 meridian, where Greenwich observatory is situated. It is a base time for zonal time shifts. LC - longitude correction is geographical longitude expressed in time value, assuming that 15° equals one mean hour. East longitude is positive, while west is negative. EOT - equation of time expresses annual discrepancy between mean and true sun time. It changes within ± 15 minutes. In fact we deal with good approximation, because we can not take into account all pecularities of Earth's movement. You can find EOT value in sun ephemeris window.