postgresql difference between two dates in days

1SELECT
2  AGE('2012-03-05', '2010-04-01'),
3  DATE_PART('year', AGE('2012-03-05', '2010-04-01')) AS years,
4  DATE_PART('month', AGE('2012-03-05', '2010-04-01')) AS months,
5  DATE_PART('day', AGE('2012-03-05', '2010-04-01')) AS days;
6 
7...  
8-- This will give you full years, month, days ... between two dates:
9
10--          age          | years | months | days
11-- -----------------------+-------+--------+------
12--  1 year 11 mons 4 days |     1 |     11 |    4

1select EXTRACT(
2	EPOCH FROM now() - (now() - INTERVAL '5 HOUR')
3)/3600
4
5-- it will return 5

1select age('2010-04-01', '2012-03-05'),
2       date_part('year',age('2010-04-01', '2012-03-05')),
3       date_part('month',age('2010-04-01', '2012-03-05')),
4       date_part('day',age('2010-04-01', '2012-03-05'));

1  -- Difference between Dec 30, 2011 08:54:55 and  Dec 30, 2011 08:56:10 in minutes
2  SELECT (DATE_PART('day', '2011-12-30 08:56:10'::timestamp - '2011-12-30 08:54:55'::timestamp) * 24 * 60 + 
3               DATE_PART('hour', '2011-12-30 08:56:10'::timestamp - '2011-12-30 08:54:55'::timestamp)) * 60 +
4               DATE_PART('minute', '2011-12-30 08:56:10'::timestamp - '2011-12-30 08:54:55'::timestamp);
5  -- Result: 1
6 
7  -- Time only
8  SELECT DATE_PART('hour', '08:56:10'::time - '08:54:55'::time) * 60 +
9              DATE_PART('minute', '08:56:10'::time - '08:54:55'::time);
10  -- Result: 1

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;