sub by_hour{
if ($a =~ /(\d\d):(\d\d)/){
        $h_a = $1;
        $mn_a = $2;
    }
if ($b =~ /(\d\d):(\d\d)/){
        $h_a = $1;
        $mn_a = $2;
    }
if ($h_a < $h_b){
        -1;
    }
if ($h_a > $h_b){
        1;
    }
if ($h_a == $h_b){
    
    if ($mn_a < $mn_b){
        -1;
    }
    if ($mn_a > $mn_b){
        1;
    }
    if ($mn_a == $mn_b){
        0;
    }
}
}