timestamp issue

Hello all,

I am running PostgreSQL 9.0.2 on Mac (10.6.6)

This is what I get in the psql terminal:

postgres=# SELECT version();
version
------------------------------------------------------------------------------------------------------------------------------------
PostgreSQL 9.0.2 on x86_64-apple-darwin, compiled by GCC i686-apple-darwin10-gcc-4.2.1 (GCC) 4.2.1 (Apple Inc. build 5664), 64-bit
(1 row)

postgres=# SELECT now() + interval '7 days' AS week_ahead;
week_ahead
----------------------------------
2011-02-24 10:39:07.071655+05:30
(1 row)

postgres=# SELECT now() - interval '7 days' AS week_behind;
week_behind
----------------------------------
2011-02-10 10:39:21.848049+05:30
(1 row)

postgres=# SELECT now() + '7 days' AS week_ahead;
week_ahead
----------------------------------
2011-02-24 10:39:29.951931+05:30
(1 row)

postgres=# SELECT now() - '7 days' AS week_behind;
ERROR: invalid input syntax for type timestamp with time zone: "7 days"
LINE 1: SELECT now() - '7 days' AS week_behind;
^

Now, if '+' operator works with '7 days', why dint the '-' operator? Is this intentional or a bug?

--
Regards,
Sachin Srivastava
EnterpriseDB, the Enterprise PostgreSQL company.