PostGre: Calculate number of overlapping days in two date ranges

fn_OverlappingDateRangesDays.sql

DROP FUNCTION IF EXISTS fn_OverlappingDateRangesDays(
	 __firstStart timestamp without time zone 
	,__firstEnd timestamp without time zone
	,__secondStart timestamp without time zone 
	,__secondEnd timestamp without time zone);

CREATE OR REPLACE FUNCTION fn_OverlappingDateRangesDays(
  	 __firstStart timestamp without time zone 
	,__firstEnd timestamp without time zone
	,__secondStart timestamp without time zone 
	,__secondEnd timestamp without time zone
 ) 
        RETURNS integer 
AS $$
DECLARE
	__maxStart timestamp without time zone;  
	__minEnd timestamp without time zone;  
	__interval int; 
BEGIN
	IF __firstStart IS NULL OR __firstEnd IS NULL OR __secondStart IS NULL OR __secondEnd IS NULL THEN 
		-- RETURN 0
		RETURN NULL;
	END IF;

	IF __firstEnd < __firstStart THEN
		RETURN 0;
	END IF;

	
	IF __secondEnd < __secondStart THEN 
		RETURN 0;
	END IF; 
		
	
	__maxStart := __secondStart; 
	__minEnd := __secondEnd; 
	
	
	IF __firstStart > __secondStart THEN 
		__maxStart := __firstStart; 
	END IF;
	
	IF __firstEnd < __secondEnd THEN 
		__minEnd :=  __firstEnd ;
	END IF;
	
	-- PRINT __maxStart 
	-- PRINT __minEnd 
	
	-- __interval := DATEDIFF(DAY, __maxStart, __minEnd)  + 1; 
	-- __interval := {fn timestampdiff(SQL_TSI_DAY, __maxStart, __minEnd)} + 1;
	__interval := EXTRACT(day from __minEnd - __maxStart) + 1;
	
	IF __interval < 0 THEN 
		__interval := 0; 
	END IF;
	
	-- PRINT __interval 
	RETURN __interval;
END;
$$ LANGUAGE plpgsql STRICT;