I guess this is due to the fact that as databases have a type for dates or dateTimes this is the default type for this info. I use them and sometimes features provided by dates (in sql or java are really useful). Seam gean generates that Pojos with that properties as java.util.date.
AFAIK there is no way to assign it directly to a Long and you should define a Date type variable and then use yourdate.getTime(), that returns you the value in miliseconds since 1970, to set your property.
I agree with you that it would be useful to be able to define which of the two data type you would like to work with.