The accuracy of the Ingleside sundial in San Francisco varies throughout the year. Its longitudinal location means it runs anything up to 24 minutes behind Pacific Standard Time for most of the year, being slowest around February 11th.
Ignoring the hour difference caused by daylight saving time, the Ingleside sundial is within a minute or so of clock time from around September 26th to October 1st and again around December 1st to December 5th. During October and November it is a few minutes ahead.
The Equation of Time explains these fluctuations, caused by the Earth’s axial tilt and its elliptical orbit around the sun. Because the orbital path is not perfectly circular, the Earth is closer to the sun at some points, causing it to speed up, lengthening the solar day between two solar noons.
