"The equation of time is probably the most fascinating complication in this outstanding model. Its
purpose is to indicate the difference in minutes between the variable solar time shown by a sundial and the constant mean time of clocks and watches. For practical reasons, mankind has divided each year into 365 and a quarter days, each day into 24 hours, and the hours into 60 minutes each. However, solar noon, the exact time when the sun reaches its zenith, does not generally correspond to the legal standard time shown by watches. In actual fact, solar time and mean time coincide just four times a year on April 15th, June 14th, September 1st and December 24th. For the rest of the year, the difference between solar and mean time varies from minus 16 minutes to plus 16 minutes."
Great read! So a regular watch is actually +/-16 minutes wrong during a year (in telling the actual day/night cycle that is..the real day)