use Time::Local;
...
for my $row (@rows) {
    my @l = reverse split /-/, $row->{start_date};
    $l[1]--; # months range is 0..11
    my $key = timelocal(0,0,0,@l);
    my $line = join("\t", $row->{id_number}, $row->{start_time}); # whatever formatting

    if(split/:/,$row->start_time)[0] > 9) {
        push @{$hash{$key-86400}}, $line;
    } else {
        push @{$hash{$key}}, $line;
    }
}
foreach my $key (sort %hash) {
    my @l = (localtime $key)[5,4,3];
    $l[1]++; # month...
    print join('-', @l), "\n";
    print $_,"\n" for @{$hash{$key}};
}