Add timezone information to the date time select labels

app/views/admin/broadcast_messages/_form.html.haml

@@ -27,11 +27,11 @@
     .col-sm-10
       = f.color_field :font, class: "form-control"
   .form-group
-    = f.label :starts_at, class: 'control-label'
+    = f.label :starts_at, _("Starts at (UTC)"), class: 'control-label'
     .col-sm-10.datetime-controls
       = f.datetime_select :starts_at, {}, class: 'form-control form-control-inline'
   .form-group
-    = f.label :ends_at, class: 'control-label'
+    = f.label :ends_at, _("Ends at (UTC)"), class: 'control-label'
     .col-sm-10.datetime-controls
       = f.datetime_select :ends_at, {}, class: 'form-control form-control-inline'
   .form-actions