CREATE OR REPLACE FUNCTION datediff (diffType Character Varying(15), date1 Date, date2 Date) RETURNS integer AS $$
DECLARE
	YEAR_CONST Character Varying(15) := 'year';
	MONTH_CONST Character Varying(15) := 'month';
	DAY_CONST Character Varying(15) := 'day';

	diffInInterval Interval;
	diffInDoublePrecision Double Precision := 0;
	diffInInteger Integer := 0;
	dateTemp Date;
BEGIN
	
	diffInInterval := age(date2, date1);

	IF lower($1) = lower(YEAR_CONST) THEN
		diffInDoublePrecision := date_part('Year', diffInInterval);
	ELSEIF lower($1) = lower(MONTH_CONST) THEN
		diffInDoublePrecision := (date_part('Year', diffInInterval) * 12) + date_part('Month', diffInInterval);
	ELSEIF lower($1) = lower(DAY_CONST) THEN
		diffInDoublePrecision := endDate - startDate;
	END IF;

	diffInInteger := CAST(diffInDoublePrecision as Integer);
	RETURN diffInInteger;
END;
$$ LANGUAGE plpgsql;

Getting the difference between two dates in Postgresql. The difference can be in days, months or years.

For difference in days, months or years pass day, month or year as the first argument

Difference between two dates in Postgresql

SQL