Why the following.......?

Dear netters, I am trying to calculate the date(13 months back) by do the following sql and informix is returning the following error. Anyone has idea why it is so? Thanks in advance. Brian Li ------------------------------------------------------------ select datetime(1996-05-31) year to day - interval(1-1) year to month from tbsctupd where subsystem_id_ind = 'j' and subfunction_code = 'date'; # ^ # 1267: The result of a datetime computation is out of range. #