This is a normal feature of the date attribute. the storage is based on a unix timestamp. As you can see in http://ez.no/doc/ez_publish/technical_manual/4_0/reference/datatypes/date : "The valid input range is 01.01.1970 - 19.01.2038."
So it's a problem for a birthday date for example...
If you really need a Date to store your attribute (ie you need some date formats or transformations in your templates), you should have a look to the birthday datatype : http://ez.no/developer/contribs/datatypes/birthday_datatype. i don't know if it's ok for the last eZ versions...
