The property inspector for the checkbox.
in design view, select the checkbox, then go to the property inspector and click the dynamic button.
the date picker format is set in the date picker options:
change the format by editing the format string:
if you want to display that in the date picker as mm/dd/yyyy edit the format string to:
the date that gets stored in the data base must in the yyyy-mm-dd format though, so you will need to add code to reformat the date that is submitted.
for example add this at line 1 for a form element named "startDate" you will need to adjust accordingly if you have a different element name:
<?php if(isset($_POST['startDate'])) $_POST['startDate'] = date("Y-m-d", strtotime($_POST['startDate'])) ?>