extract('epoch' from age()) returning wrong number of seconds

The forth column in this query is returning the wrong value.

The last two columns in this query should return the same value, 5270400 (the number of seconds in 61 days). Using simple subtraction works fine, but using the age() function it returns 5184000 seconds (60 days). This same behavior can be found using other start/end dates and interval lengths, sometimes age() returns too many seconds, sometimes too few. I did not get any failures below 60 days, but I did not do an exhaustive test of all interval values and a range of start/end dates.

> select version();
version
----------------------------------------------------------------------------------------------
PostgreSQL 9.2.6 on x86_64-unknown-linux-gnu, compiled by gcc (Debian 4.7.2-5) 4.7.2, 64-bit
(1 row)

Time: 41.898 ms
> select extract('epoch' from ('2013-12-31'::date) - ('2013-12-31'::date - '5184000 seconds'::interval)) as right_answer_subtraction_60_days
,extract('epoch' from age (('2013-12-31'::date) , ('2013-12-31'::date - '5184000 seconds'::interval))) as right_answer_age_60_days
,extract('epoch' from ('2013-12-31'::date) - ('2013-12-31'::date - '5270400 seconds'::interval)) as right_answer_subtraction_61_days
,extract('epoch' from age (('2013-12-31'::date) , ('2013-12-31'::date - '5270400 seconds'::interval))) as wrong_answer_age_61_days;
right_answer_subtraction_60_days | right_answer_age_60_days | right_answer_subtraction_61_days | wrong_answer_age_61_days
----------------------------------+--------------------------+----------------------------------+--------------------------
5184000 | 5184000 | 5270400 | 5184000
(1 row)