use vars qw/ $withDateTime /;
    if( $withDateTime  ){
        use if $withDateTime,qw/ DateTime::Span /;
        use if $withDateTime,qw/ DateTime::Format::Strptime /;
#~         use DateTime::Span;
#~         use DateTime::Format::Strptime;
        my $strp = DateTime::Format::Strptime->new(
#~                 29.10.2008 13:00:00
#~                 %d.%m.%Y   %T
            pattern => '%d.%m.%Y %T',
        );
        
        my $span_a = DateTime::Span->from_datetimes(
            start => $strp->parse_datetime($start_a_str),
            end => $strp->parse_datetime($end_a_str),
        );
        my $span_b = DateTime::Span->from_datetimes(
            start => $strp->parse_datetime($start_b_str),
            end => $strp->parse_datetime($end_b_str),
        );
        my $set = $span_a->intersection( $span_b );
        my $dur = $set->duration;
        my $thours = 0;
        $thours += $dur ->years *365*24;
        $thours += $dur ->months *30*24;
        $thours += $dur ->weeks *7*24;
        $thours += $dur ->days *24;
        $thours += $dur ->hours ;
        $thours += $dur ->minutes  / 60 ;
        $thours += $dur ->seconds  / 60 / 60 ; # irrelevant
        $thours = sprintf '%.f', $thours; # round 
        $hash{$wds} +=  $thours if $thours;
        next STRING;
    }