Create two unbound controls on a form. Name them FromDT and ThruDT. Set the control's Format property to Short Date. Then in the query, instead of the prompt, use
Where ExpenseDT Between Forms!yourformname!FromDT AND Forms!yourform!ThruDT
Using form field references also allows you to validate the date entries before opening the report should you want to but at least telling Access that the controls are dates by setting the format property, Access will ensure that the values re at least valid dates. They may make no sense but they will be dates.