Date of 01-01-9999?

We have a canned application which has somehow entered a value of '01-01-9999' in an Oracle 7.0 database (HP-UX). That is, we can do a

SELECT DATE_FIELD
FROM MY_TABLE; DATE_FIELD

or

SELECT TO_CHAR('DD-MM-YYYY', DATE_FIELD) FROM MY_TABLE; DATE_FIELD

Oracle dates are normally limited to a top end of the year 4713 or so.

How did this date field acquire this value? We can't insert a row like this, or update an existing row.

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