Note that the milliseconds must be converted to seconds before they can be applied as an interval to the epoch (01/01/1970).
Table "public.candle_l"
Column |     Type     | Modifiers
--------+--------------+-----------
ticker | character(8) | not null
minute | bigint       | not null
open   | numeric      | not null
high   | numeric      | not null
low    | numeric      | not null
close  | numeric      | not null
volume | bigint       | not null

SELECT ticker, minute, '1970-01-01 00:00:00 GMT'::timestamp + ((minute/1000)::text)::interval, close FROM candle_l where ticker='BARC' ORDER BY 2;

ticker   |    minute     |      ?column?       |  close  
----------+---------------+---------------------+---------
BARC     | 1306220400000 | 2011-05-24 07:00:00 |   266.7
BARC     | 1306220460000 | 2011-05-24 07:01:00 |   266.8
BARC     | 1306220520000 | 2011-05-24 07:02:00 |     266
BARC     | 1306220580000 | 2011-05-24 07:03:00 |  265.95
BARC     | 1306220640000 | 2011-05-24 07:04:00 |  266.25
BARC     | 1306220700000 | 2011-05-24 07:05:00 |   266.6
BARC     | 1306220760000 | 2011-05-24 07:06:00 |  267.15