diff --git a/www/conservancy/apps/assignment/forms.py b/www/conservancy/apps/assignment/forms.py
index 8777e0b4857fc36018b98894832587f60447ed31..288dc75aa54bcfee7bf8e7da40aad497b97e6328 100644
--- a/www/conservancy/apps/assignment/forms.py
+++ b/www/conservancy/apps/assignment/forms.py
@@ -62,3 +62,6 @@ class AssignmentForm(forms.ModelForm):
     def clean_period_ends(self):
         if 'period_begins' in self.cleaned_data and 'period_ends' in self.cleaned_data and self.cleaned_data['period_begins'] and self.cleaned_data['period_ends'] and self.cleaned_data['period_begins'] > self.cleaned_data['period_ends']:
             raise ValidationError('End of period is before start')
+
+        if self.cleaned_data['period_end_type'] == 'a specific past date' and not self.cleaned_data['period_ends']:
+            raise ValidationError('This field is required')
diff --git a/www/conservancy/templates/assignment/assignment_form.html b/www/conservancy/templates/assignment/assignment_form.html
index ea13b9a12d9a9069d46574d6cf4322678caabb45..d7c2cc1a6c58da9548cdc4b3f135047d38ed3262 100644
--- a/www/conservancy/templates/assignment/assignment_form.html
+++ b/www/conservancy/templates/assignment/assignment_form.html
@@ -24,7 +24,8 @@
    // End date field should be shown only when "a specific past date" is selected.
    const form = document.querySelector('#assignment-form');
    const past_date_label = document.querySelector('label[for=id_period_ends]');
-   const past_date_field = document.querySelector('#assignment-form > p:nth-child(11)');
+   const past_date_field = document.querySelector('#id_period_ends');
+   const past_date_container = past_date_field.parentElement;
    form.addEventListener('change', togglePastDate);
    togglePastDate();  // Run change handler once to initialise form.
 
@@ -33,10 +34,12 @@
 
    function togglePastDate() {
        if (form['period_end_type'].value === 'all future contributions') {
-           past_date_field.style.display = 'none';
+           past_date_container.style.display = 'none';
+           past_date_field.required = false;
        }
        else {
-           past_date_field.style.display = '';
+           past_date_container.style.display = '';
+           past_date_field.required = true;
        }
    }
   </script>