I'm not sure either. I think if your field is set to the date/time data type it won't allow u to enter it like that. It might work if it's set to a number data type? But that might defeat the purpose of having a date.
Is there a reason u need the users to enter the date like that? Another option would be to allow them to enter dates in the usual method and just change the format of the field to "yyyyddmm". This would display the date in the format u want in any forms and reports, but still allow Access to do it's normal date validation checking.