clip to end of month instead of overflow to next

ShimmerFairy++ for the suggestion. This simplifies the logic a bit.

src/core/Temporal.pm

@@ -373,13 +373,9 @@ my class DateTime does Dateish {
                 $month += $amount;
                 $year += floor(($month - 1) / 12);
                 $month = ($month - 1) % 12 + 1;
-                my $day = $!day;
                 # If we overflow on days in the month, rather than throw an
-                # exception, we just clip to the 1st of the next month
-                if $day > $.days-in-month($year, $month) {
-                    $day = 1;
-                    $month++;
-                }
+                # exception, we just clip to the last of the month
+                my $day = $!day min $.days-in-month($year, $month);
                 $date = Date.new(:$year, :$month, :$day);
                 succeed;
             }
@@ -612,13 +608,9 @@ my class Date does Dateish {
                 $month += $amount;
                 $year += floor(($month - 1) / 12);
                 $month = ($month - 1) % 12 + 1;
-                my $day = $!day;
                 # If we overflow on days in the month, rather than throw an
-                # exception, we just clip to the 1st of the next month
-                if $day > $.days-in-month($year, $month) {
-                    $day = 1;
-                    $month++;
-                }
+                # exception, we just clip to the last of the month
+                my $day = $!day min $.days-in-month($year, $month);
                 $date = Date.new(:$year, :$month, :$day);
                 succeed;
             }