Oracle will always store DATE data in a packed binary format. This is handy for computers to use, but incomprehensible to applications. It is impossible to change the way Oracle stores a date.
It is the job of the client application, like SQL*Plus, to turn this binary data into a format that is comprehensible to the user. Oracle's tools will use NLS_DATE_FORMAT to determine how to display dates by default, though you can always use TO_CHAR to do an explicit format. NLS_DATE_FORMAT can be set at the session level, at the client level by setting an environment variable, or at the database level, by changing the initialization parameter. Individual sessions, however, can always override client and database level settings. Additionally, non-Oracle tools will generally not use NLS_DATE_FORMAT and will end up using their own date format.
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