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=3D# SELECT version();
                                                              version=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20
---------------------------------------------------------------------------=
---------------------------------------------------------
 PostgreSQL 9.0.2 on x86_64-apple-darwin, compiled by GCC i686-apple-darwin=
10-gcc-4.2.1 (GCC) 4.2.1 (Apple Inc. build 5664), 64-bit
(1 row)

postgres=3D# SELECT now() + interval '7 days' AS week_ahead;
            week_ahead=20=20=20=20=20=20=20=20=20=20=20=20
----------------------------------
 2011-02-24 10:39:07.071655+05:30
(1 row)

postgres=3D# SELECT now() - interval '7 days' AS week_behind;
           week_behind=20=20=20=20=20=20=20=20=20=20=20=20
----------------------------------
 2011-02-10 10:39:21.848049+05:30
(1 row)

postgres=3D# SELECT now() + '7 days' AS week_ahead;
            week_ahead=20=20=20=20=20=20=20=20=20=20=20=20
----------------------------------
 2011-02-24 10:39:29.951931+05:30
(1 row)

postgres=3D# 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 th=
is intentional or a bug?

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