SQL & PL/SQL

So i was tinkering and...

ALTER DATABASE SET TIME_ZONE = 'Europe/Zurich';

this gave me a 30079 error

*Cause:    An attempt was made to alter database timezone with

           TIMESTAMP WITH LOCAL TIME ZONE column in the database.

*Action:   Either do not alter database timezone or first drop all the

           TIMESTAMP WITH LOCAL TIME ZONE columns.

Any reasons/insight into this?

Thanks