Automatic Departure Date

I tried to set automatic departure date whit something like this:

<script>
jQuery(function($){
var start = $(‘input[name=”check_in_date”]’).first();
var end = $(‘input[name=”check_out_date”]’).first();

start.on(‘change’, function() {
var start_date = $(this).datepicker(‘getDate’);
start_date.setDate(start_date.getDate() + 7);
end.datepicker(‘option’, ‘minDate’, start_date);
});
});
</script>

…but, it doesn’t works.

What I really want is:
Min Reservation: 7 days.
Reservation: Saturday to Saturday.
Possibility to set min reservation only for july and August.

Thanks
Bye
_G