Month interval bug

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POSTGRESQL BUG REPORT TEMPLATE
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Your name : Andrew V. Gromozdin
Your email address : ag(at)df(dot)ru

System Configuration
---------------------
Architecture (example: Intel Pentium) : Intel Pentium

Operating System (example: Linux 2.0.26 ELF) : Linux 2.0.38 ELF

PostgreSQL version (example: PostgreSQL-6.4.2) : PostgreSQL-6.4.2

Compiler used (example: gcc 2.8.0) : gcc 2.7.2.3

Please enter a FULL description of your problem:
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QUERY: select '09/01/1999 00:00:01.00 MSD'::datetime +
'1month'::timespan as err
or_date;

error_date
---------------------------------
Fri\ Oct\ 01\ 00:00:01\ 1999\ MSD
(1 row)

QUERY: select '09/01/1999 00:00:01.00 MSD'::datetime +
'2month'::timespan as err
or_date;

error_date
---------------------------------
Sun\ Oct\ 31\ 23:00:01\ 1999\ MSK
(1 row)

QUERY: select '09/01/1999 00:00:01.00 MSD'::datetime +
'3month'::timespan as err
or_date;

error_date
---------------------------------
Tue\ Nov\ 30\ 23:00:01\ 1999\ MSK
(1 row)

QUERY: select '09/01/1999 00:00:01.00 MSD'::datetime +
'4month'::timespan as err
or_date;

error_date
---------------------------------
Fri\ Dec\ 31\ 23:00:01\ 1999\ MSK
(1 row)

QUERY: select '09/01/1999 00:00:01.00 MSD'::datetime +
'5month'::timespan as err
or_date;

error_date
---------------------------------
Mon\ Jan\ 31\ 23:00:01\ 2000\ MSK
(1 row)

Please describe a way to repeat the problem. Please try to provide a
concise reproducible example, if at all possible:
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run this query from psql

If you know how this problem might be fixed, list the solution below:
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I think it's because of unnormal defined MONTH interval

--
Sincerely Your
Andrew V. Gromozdin, DataForce ISP, Moscow, Russia