Date/Time format models can be used in the functions TO_CHAR (datetime)/ TO_DATE/ TO_TIMESTAMP and in ETL commands IMPORT/ EXPORT. If no date/time format is specified, the current default format is used. This default format is defined in the NLS_DATE_FORMAT and NLS_TIMESTAMP_FORMAT session parameters. It can be changed for the current session using ALTER SESSION or for the entire database using ALTER SYSTEM.