data yang sudah ada tidak boleh di input lagi

mau tanya,bagaimana buat validasi ajax/javascript bahwa tanggal tersebut sudah ada(already exist).

<form  class="form-horizontal"  method="post" action="<?php echo base_url(); ?>activity/add">
<input type="hidden" class="form-control" name="nik" value="<?php echo $this->session->userdata('nik')?>">
<div class="input-group date date-picker" data-date-format="yyyy-mm-dd">
	<input type="text"  class="form-control" readonly name="start" required>
	<span class="input-group-addon">
	<span class="glyphicon glyphicon-calendar"></span>
	</span>
</div>
<button type="submit" class="btn btn-success"><i class="glyphicon glyphicon-plus"></i> new activity</button>
</form>