Equation of time from PVCDROM.
PVCDROM is a website by Solar Power Lab at Arizona State University (ASU)
dayofyear (numeric) –
equation_of_time (numeric) – Difference in time between solar time and mean solar time in minutes.
Soteris A. Kalogirou, “Solar Energy Engineering Processes and Systems, 2nd Edition” Elselvier/Academic Press (2009).