I have changed my FIXED_DATE parameter to '01.01.2000'.
After changing this parameter I got SYSDATE as 01.01.2000
which was as expected.

Now I want to change this back to normal behavior of SYSDATE. Even though I re-started the database I don't get SYSDATE as current date, but still I get SYSDATE as 01.01.2000.

How do I change this back to get current date as SYSDATE?

What is the default value for this parameter. Which value I have to set for this parameter in init.ora so that it will get back to normal behavior.