postgres time difference in minutes

1   CREATE OR REPLACE FUNCTION DateDiff (units VARCHAR(30), start_t TIME, end_t TIME) 
2     RETURNS INT AS $$
3   DECLARE
4     diff_interval INTERVAL; 
5     diff INT = 0;
6   BEGIN
7     -- Minus operator for TIME returns interval 'HH:MI:SS'  
8     diff_interval = end_t - start_t;
9 
10     diff = DATE_PART('hour', diff_interval);
11 
12     IF units IN ('hh', 'hour') THEN
13       RETURN diff;
14     END IF;
15 
16     diff = diff * 60 + DATE_PART('minute', diff_interval);
17 
18     IF units IN ('mi', 'n', 'minute') THEN
19        RETURN diff;
20     END IF;
21 
22     diff = diff * 60 + DATE_PART('second', diff_interval);
23 
24     RETURN diff;
25   END;
26   $$ LANGUAGE plpgsql;