For a given day, and optionally C<month> and C<year> if they are
different from those specified in C<new()>, provide the day of week
For a given day, and optionally C<month> and C<year> if they are
different from those specified in C<new()>, provide the day of week
=item add_days($days, $delta, $day, [$month], [$year])
=item add_days($days, $delta, $day, [$month], [$year])