Fix for passing in a Carbon 3 object

Carbon 3 made some breaking changes here. (Specifically from the Laravel 11 upgrade)

If we're using Carbon, let's use the diffInDays method to get the difference in days.

Otherwise, we're probably using DateTime, so we'll use the diff method.

See: 
https://carbon.nesbot.com/docs/#api-carbon-3
https://www.php.net/manual/en/class.dateinterval.php

This definitely isn't the ultimate answer here - but it fixes the breaking change for my SaaS.

src/Recurr/DateUtil.php

@@ -118,8 +118,17 @@ public static function getDaySetOfWeek(
         $start = clone $dt;
         $start = $start->setDate($start->format('Y'), 1, 1);
 
-        $diff  = $dt->diff($start);
-        $start = $diff->days;
+        // Carbon 3 made some breaking changes here. (Specifically from the Laravel 11 upgrade)
+        // If we're using Carbon, let's use the diffInDays method to get the difference in days.
+        // Otherwise, we're probably using DateTime, so we'll use the diff method.
+        if(class_basename($start) == 'Carbon') {
+            // Carbon 3 also returns a float here now, so we cast it to an int to be more what we expect.
+            $diff_in_days  = (int) $start->diffInDays($dt);
+            $start = $diff_in_days;
+        } else {
+            $diff  = $dt->diff($start);
+            $start = $diff->days;
+        }
 
         $set = array();
         for ($i = $start, $k = 0; $k < 7; $k++) {