how to get diff days between two dates if I consider every month is day 30 only

$start_date = Carbon::parse(date('Y-m-d', strtotime('2022-08-17')));
$end_date = Carbon::parse(date('Y-m-d', strtotime('2022-09-01')));
$diff = $start_date->diffInDays($end_date);

Days between these two dates is 15, because aug is 31 day but in my case I consider every month is 30 day, so the difference should be 14