a date data processor verifies whether an inputted date data exists or not . inputted date data is stored in an input register which is composed of three sections according to month , day and year . a first calculating unit calculates the days difference between the inputted date data and a reference date , and the results are temporarily stored . a second calculating unit then performs an inverse calculating to arrive at a date based on the previously calculated days difference . the resulting date is compared with the inputted date to complete the verification .