Date format changes in the middle of a program execution

Question

In my C code I have a series of select statements.
When I first get a session to the database I use the following command to set the date format
alter session set nls_date_format = 'MM/DD/YYYY HH24:MI:SS'
It works fine for a few queries. After a while during the program execution I see that the date format changes to 'DD-MON-YY' format. This results in a series of error in my code because I expect the date format to always be in the 'MM/DD/YYYY HH24:MI:SS' format.
Any idea why the date format should change all of a sudden in the middle of the program execution.

Answer 1

I second the idea that you should always use TO_DATE and TO_CHAR if you want to reliably convert between dates and strings.
Without seeing your code, it is hard to say why your date format is changing, but the most likely reason is that you are changing users somewhere in the code. For example:
SQL> show user
USER is "OPS$ORACLE"
SQL> SELECT sysdate FROM dual;
SYSDATE
11-JUL-2003
SQL> ALTER SESSION SET nls_date_format='dd-Mon-yyyy hh24:mi:ss';
Session altered.
SQL> SELECT sysdate FROM dual;
SYSDATE
11-Jul-2003 10:15:12
SQL> connect jtest/test
Connected.
SQL> SELECT sysdate FROM dual;
SYSDATE
11-JUL-2003TTFN
John