diff --git a/www/app.html b/www/app.html
index df1d3c6..21b8457 100644
--- a/www/app.html
+++ b/www/app.html
@@ -74,7 +74,8 @@
 
         <div class="option" id="week">
           week:
-          <input type="date" id="fDate" name="date" step="7" />
+          <input type="date" id="fDate" name="date" step="7" /><span
+            class="validity"></span>
         </div>
 
         <button type="button" onclick="kies()" id="fSubmit" disabled>
diff --git a/www/style.css b/www/style.css
index aaf405a..f762d76 100644
--- a/www/style.css
+++ b/www/style.css
@@ -167,3 +167,11 @@ tr.last td {
 tr.first td {
   padding-top: 0.5em;
 }
+
+#fDate:invalid + span::after {
+  content: ' ✖';
+}
+
+#fDate:valid + span::after {
+  content: ' ✓';
+}