Default date format

Does anyone know of a wya to change the default date format from DD-MON-YY? All the applications we're now writing have to cope with possible dates after the year 1999, and so our code has TO_CHAR(date, 'DD-MON-YYYY') or TO_DATE(char, 'DD-MON-YYYY') littered through it?
Why don't oracle have a system-wide date format, eg. in the config.ora file, so each database instance can use it's own date representation?

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