for my $date (@datesEvery15th) {
    if (Date_IsWorkDay($date)) {
	say UnixDate($date, "%Y-%m-%d");
    }
    else {
	my $nearestWorkingData = Date_NearestWorkDay($date);
	say UnixDate($nearestWorkingData, "%Y-%m-%d");
    }
}

__END__

2017-10-09
2017-10-24
2017-11-08
2017-11-23
2017-12-08
2017-12-22