The equation of time is a formula used to compensate for the earth's elliptical orbit around the sun and its axial tilt when calculating solar time. Graphically, it appears as:
It can be approximated by the following formula:
E = 9.87 * sin (2B) - 7.53 * cos (B) - 1.5 * sin (B)
Where:
B = 360 * (N - 81) / 365
Where:
N = day number, January 1 = day 1