Ok - I see what you are saying. But, I can't change the field type because I was using an existing field -- i think that boolean is irrelevant (and confusing).
So then - i tried to a new field and then using the field mapper -- but that didn't work either.
Then, i remembered this: the due date field is new since this geoevent service was created. and even though i had added it to the geoevent definitions i was using for the inputs, etc --- the output service (Update a Feature Service) didn't know about my new duedate field. BUT - i don't know how to fix that without recreating the output service.
Anyways -- removing this output, deleting the corresponding output service, recreating it that service, then reconfiguring my geoevent service fixed the use. The field calculator as originally designed worked as expected. oy!