When you set it to null in the javascript it goes to the default value set in the form element. To make it start blank, you could use:
<input type="text" class="form-control" name="DOB" id="datepicker_1" placeholder="Date of Birth" maxlength="10" value="<?php echo((ValidatedField("healthscreeningform_984","DOB")?ValidatedField("healthscreeningform_984","DOB"):"")); ?>" aria-describedby="basic-addon2" required>