PURPOSE: To provide the managing method for a year and date key being useful for processing a data file in which the year and date keys of both the years are mixed especially in the case the Christian Era year is transferred from the 1900 year level to the 200 year level, with regard to the managing method for the year and date key in a data processing system for processing the data file having the year and date key of an abbreviated form in which the upper two digits of the Chastian Era year are omitted.

CONSTITUTION: When a key file 3 of a year and date key is prepared with regard to a data file 1, or data are sorted by using the year and date key, year data in the year and date key is compared with a prescribed threshold, and based on its result, a value of two digits 19 or 20 is added to the higher coder of the year data, and the year data are restored to the Christian Era year of four digits. By the year and date key containing this restored year data, the key file 3 is prepared or the data is sorted.