diff --git a/www/conservancy/templates/assignment/assignment_form.html b/www/conservancy/templates/assignment/assignment_form.html
index 753cd42760cad40ad5b769b3e57ad5acb698ea84..ea13b9a12d9a9069d46574d6cf4322678caabb45 100644
--- a/www/conservancy/templates/assignment/assignment_form.html
+++ b/www/conservancy/templates/assignment/assignment_form.html
@@ -10,11 +10,34 @@
 
     <p>If you have any questions about assigning your copyright to Conservancy, please don't hesitate to email us at <a href="mailto:info@sfconservancy.org">info@sfconservancy.org</a>.</p>
 
-    <form action="." method="post" class="mw7">
+    <form id="assignment-form" action="." method="post" class="mw7">
       {% csrf_token %}
       {{ form.as_p }}
 
-      <p><button type="submit" class="ph3 pv2">Next</button></p>
+      <p><button type="submit" class="ph3 pv2">Submit</button></p>
     </form>
   </div>
+
+  <script>
+   'use strict';
+
+   // 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)');
+   form.addEventListener('change', togglePastDate);
+   togglePastDate();  // Run change handler once to initialise form.
+
+   // Text "(if applicable)" isn't relevant with JS enabled.
+   past_date_label.innerHTML = past_date_label.innerHTML.replace(' (if applicable)', '');
+
+   function togglePastDate() {
+       if (form['period_end_type'].value === 'all future contributions') {
+           past_date_field.style.display = 'none';
+       }
+       else {
+           past_date_field.style.display = '';
+       }
+   }
+  </script>
 {% endblock %}