Validity Dates (Start Date & End Date) are not available in the approval attribute for BPA

Hi, we are trying to setup approval based on the validity dates of BPA, but the attributes are not available in the conditions to be used. We are able to see that start date modified and end date modified are available whereas the start date and end date are not available. Please advise if this the intended behavior, we would want the BPA to be rejected if there is no start date or end date in the agreement, understand using sandbox we can make these fields mandatory but is there any feasibility to use these attributes in the approval workflow.