It seems that portal filters need a good refresh once you change the value of a global field (or two).
Are you using FileMaker 14? IF so, you can name the portal (Inspector / Position Tab, top area). Then use a script that has the step "Refresh Portal [<<portal name>>]"
You'd have to then run this via a button on the layout that the user clicks on after updating the date fields, or run it as a script trigger on the fields. Script triggers will run before or after an event, so I would use the onExit trigger. To assign a script trigger, right-click on the field in layout mode, Choose "Set Script Triggers" and choose the onExit one.