Change the title of date_popup filed in custom form
Just use the following code:

'#date_label_position' =&gt; 'none',


your code :

$form['filter']['start_date'] = array(
    '#type' =&gt; 'date_popup',
    '#date_format' =&gt; 'd-m-Y',
    '#title' =&gt; t('Start date'),
    '#date_label_position' =&gt; 'none',
  );

  $form['filter']['end_date'] = array(
    '#type' =&gt; 'date_popup',
    '#date_format' =&gt; 'd-m-Y',
    '#title' =&gt; t('End date'),
    '#date_label_position' =&gt; 'none',
  );