@fredmarks What about this. You add month to last_renewed date if it's not in future you just add month to actual date, in else part just return date base on last_renewed date + 30 days.
use Carbon\Carbon;
$date = Carbon::parse('2020-01-23'); // this will be your last_renewed from DB
$date->addMonth();
if ($date->lessThan(Carbon::now())) {
echo Carbon::now()->addMonth();
} else {
echo $date;
}