how to display only time in jquery datepicker?

try a code like this:

<script>
  $('#time_input').datetimepicker({
     datepicker:false,
      format:'H:i'
  }); 
</script>

Just set dateFormat to an empty string dateFormat: ''

$('#field-start_t_a').datetimepicker({
    dateFormat: '',
    timeFormat: 'hh:mm tt'
});

(this is a little strange that you only want to set time from the datetimepicker, because you can also do $('#field-start_t_a').timepicker(); and only the time options show)

$('.picker').datetimepicker({
    dateFormat: '',
        timeFormat: 'hh:mm tt',
        **timeOnly: true**
});