The Equation of Time is a complication displaying the difference between solar time (sundial) and mean time (clock), which varies up to 16 minutes throughout the year.
The Equation of Time displays the difference between true solar time (measured by a sundial) and mean time (our standard clock time). This difference varies throughout the year by up to +16 minutes in February and -14 minutes in November, due to Earth's elliptical orbit and axial tilt.
The Equation of Time is an ultra-prestigious complication requiring deep astronomical knowledge to create—it needs a custom cam programmed with the solar deviation for every day of the year. It appeals to those fascinated by astronomy, sundials, and the highest levels of horological complexity and craftsmanship.
Patek Philippe is most famous for Equation of Time complications, often combined with perpetual calendars. Other notable makers include Breguet (which historically pioneered it), A. Lange & Söhne, Jaeger-LeCoultre, and Ulysse Nardin. These are exclusively ultra-luxury timepieces due to extreme manufacturing complexity.