Set time zone on login

Should be more reliable.

views/votes_show_unauthenticated.erb

@@ -14,4 +14,9 @@
     <input type="password" name="password" class="form-control">
   </div>
   <button type="submit" class="btn btn-primary"><%= _("Login") %></button>
+  <input type="hidden" name="timezone" value="UTC">
 </form>
+
+<script>
+document.login.timezone.value = Intl.DateTimeFormat().resolvedOptions().timeZone;
+</script>