time zone problem in Oracle 8.1.7

Initially we had Oracle 8.1.5 and we upgraded that to Oarcle 8.1.7

I have got a UNIX shell script which does the following,

1.Changes the time zone so that we scroll back to the previous day or before (two days back)
(setting the system date as the date of the previous day)
TZ=EST17EDT

2.Connects to oracle and queries the database.

When we had oracle 8.1.5 this code was working fine without any problems
but in 8.1.7 I am getting the following error,


ERROR:
ORA-01857: not a valid time zone


SP2-0306: Invalid option.
Usage: CONN[ECT] [logon] [AS {SYSDBA|SYSOPER}] | [INTERNAL]
where ::= [/][@] | /
SP2-0306: Invalid option.
Usage: CONN[ECT] [logon] [AS {SYSDBA|SYSOPER}] | [INTERNAL]
where ::= [/][@] | /
SP2-0157: unable to CONNECT to ORACLE after 3 attempts, exiting SQL*Plus
ERROR IN RETRIEVAL PROCESSING



On studying the problem I found that
Oracle 8.1.7 accepts only -12 to 13 as the time zone offset range
On specifying the offset with in this range (TZ=EST10EDT)
oracle recognises it and the connection is successful.
When I specify more than the range (value as 17) I am getting the error
and it doesn't connect to Oracle. It fails while connecting to oracle.



I wonder how it was working fine in 8.1.5 but not in 8.1.7

Can anyone please help me out in this regard.

Thanks in advance.

Have you set the NLS_DATE_FORMAT parameter in your profile of unix?, if not try to set it and then try using SQLPLUS