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