So I found the solution right after posting this, it's NOT intuitive at all! You have to go to your "custom form styles" page then edit the canned template called "Standard" even if you have never or will never use it. You then have to enable dates checkbox in that Standard template, and save it. ONLY then your <servicedate>  EZ Map fields from your docx templates will actually show up in your invoices (even though they show up in preview, this step is necessary to make them display in practice)