Leap year agnostic day of year function for Postgres PL/SQL

gistfile1.sql

CREATE OR REPLACE FUNCTION yday(time_in TIMESTAMP) RETURNS INTEGER AS $$
DECLARE
  yr INTEGER;
  mnth INTEGER;
  yday INTEGER;
BEGIN
  -- Ensure March 1st and after always have the same day of year across leap-years
  -- by adding 1 to the day of year for months after Feb in non-leap years
  yday := EXTRACT(doy FROM time_in);
  yr := EXTRACT(year FROM time_in);
  mnth := EXTRACT(month FROM time_in);
  IF yr % 4 > 0 and mnth > 2 THEN
    yday := yday + 1;
  END IF;
  RETURN yday;
END;
$$ LANGUAGE plpgsql;

This function not includes adjustments introduced by Gregorian calendar.

Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100, but these centurial years are leap years if they are exactly divisible by 400. For example, the years 1700, 1800, and 1900 are not leap years, but the years 1600 and 2000 are.