From the Oracle DOCS

NLS_DATE_FORMAT
Parameter type String

Syntax NLS_DATE_FORMAT = "format"

Default value Derived from NLS_TERRITORY

Parameter class Dynamic: ALTER SESSION

Range of values Any valid date format mask but not exceeding a fixed length




NLS_DATE_FORMAT specifies the default date format to use with the TO_CHAR and TO_DATE functions. The default value of this parameter is determined by NLS_TERRITORY.

The value of this parameter can be any valid date format mask, and the value must be surrounded by double quotation marks. For example:

NLS_DATE_FORMAT = "MM/DD/YYYY"

You can change the session but on system..