missing from solution: rollup field -- _date field, HOW?

I have two rollup fields, new_invoicebalance & new_orderbalance.  When I export a solution it includes new_invoicebalance_date & new_orderbalance_date as missing; when I import it halts because they are missing.

How can the two be separated (you can't add the _date fields separately) and how do I get them back together?

I think it actually says they are needed by a form which doesn't have them.

TIA