PostgreSQL DATEDIFF

Besides a separate expression to calculate the datetime difference for each time unit, you can use a function similar to SQL Server DATEDIFF function. 

PostgreSQL:

   CREATE OR REPLACE FUNCTION DateDiff (units VARCHAR(30), start_t TIMESTAMP, end_t TIMESTAMP) 
     RETURNS INT AS $$
   DECLARE
     diff_interval INTERVAL; 
     diff INT = 0;
     years_diff INT = 0;
   BEGIN
     IF units IN ('yy', 'yyyy', 'year', 'mm', 'm', 'month') THEN
       years_diff = DATE_PART('year', end_t) - DATE_PART('year', start_t);
 
       IF units IN ('yy', 'yyyy', 'year') THEN
         -- SQL Server does not count full years passed (only difference between year parts)
         RETURN years_diff;
       ELSE
         -- If end month is less than start month it will subtracted
         RETURN years_diff * 12 + (DATE_PART('month', end_t) - DATE_PART('month', start_t)); 
       END IF;
     END IF;
 
     -- Minus operator returns interval 'DDD days HH:MI:SS'  
     diff_interval = end_t - start_t;
 
     diff = diff + DATE_PART('day', diff_interval);
 
     IF units IN ('wk', 'ww', 'week') THEN
       diff = diff/7;
       RETURN diff;
     END IF;
 
     IF units IN ('dd', 'd', 'day') THEN
       RETURN diff;
     END IF;
 
     diff = diff * 24 + DATE_PART('hour', diff_interval); 
 
     IF units IN ('hh', 'hour') THEN
        RETURN diff;
     END IF;
 
     diff = diff * 60 + DATE_PART('minute', diff_interval);
 
     IF units IN ('mi', 'n', 'minute') THEN
        RETURN diff;
     END IF;
 
     diff = diff * 60 + DATE_PART('second', diff_interval);
 
     RETURN diff;
   END;
   $$ LANGUAGE plpgsql;

How to Use PostgreSQL DATEDIFF Function

The syntax is similar to SQL Server DATEDIFF, but you have to specify a time unit (second, minute etc. and their abbreviations) as a string literal in PostgreSQL, for example:

    -- Difference between Dec 30, 2011 08:54:55 and  Dec 30, 2011 08:56:10 in seconds
  SELECT DATEDIFF('second', '2011-12-30 08:54:55'::timestamp, '2011-12-30 08:56:10'::timestamp);
  -- Result: 75

PostgreSQL DATEDIFF Function for TIME Only

You can have another function that operates on time data types only. PostgreSQL supports overloaded functions having the same name, but different data types of parameters:

   CREATE OR REPLACE FUNCTION DateDiff (units VARCHAR(30), start_t TIME, end_t TIME) 
     RETURNS INT AS $$
   DECLARE
     diff_interval INTERVAL; 
     diff INT = 0;
   BEGIN
     -- Minus operator for TIME returns interval 'HH:MI:SS'  
     diff_interval = end_t - start_t;
 
     diff = DATE_PART('hour', diff_interval);
 
     IF units IN ('hh', 'hour') THEN
       RETURN diff;
     END IF;
 
     diff = diff * 60 + DATE_PART('minute', diff_interval);
 
     IF units IN ('mi', 'n', 'minute') THEN
        RETURN diff;
     END IF;
 
     diff = diff * 60 + DATE_PART('second', diff_interval);
 
     RETURN diff;
   END;
   $$ LANGUAGE plpgsql;

For example, you can call this function as:

  -- Difference between 08:54:55 and 08:56:10 in seconds
  SELECT DATEDIFF('second', '08:54:55'::time, '08:56:10'::time);
  -- Result: 75

Link:

http://www.sqlines.com/postgresql/how-to/datediff