persian-calendar
This Go Module can be used for converting dates between Persian and Gregorian calendars. It is based on the work of English mathematician, KAZIMIERZ M. BORKOWSKI, which is "Using the analytical theory of the motion of the Earth around the Sun the times of the vernal (Spring) equinox has been calculated over the period from the Hijra (AD 622) to AD 3800".
Some of test fixtures for Persian leap years are imported from this repo and it's based on calculations from Geophysics Institute of University of Tehran.