sub get_first
{
    # usage: get_first("firstfile");
    my $f1=shift; # first file
    tie my @file1, 'Tie::File', "$workdir$f1" or die("Can't tie $f1: $!\n");
    my $first=@file1[0];
    untie @file1;
    my ($dow, $month, $day, $time, $year, $line)=split(/\s+/, $first);
    my $start_time=join ' ', $month,$day,$year,$time;
    return $start_time;
}

sub get_last
{
    # usage: get_last("lastfile");
    my $f2=shift; # last File
    tie my @file2, 'Tie::File', "$workdir$f2" or die("Can't tie $f2: $!\n");
    my $last=@file2[$#file2];
    untie @file2;
    my ($dow, $month, $day, $time, $year, $line)=split(/\s+/, $last);
    my $end_time=join ' ', $month,$day,$year,$time;
    return $end_time;
}