Thanks Ikegami, i have modified my code according to your suggestions. however now i got an error message when i run the code:"use of uninitialized value in negation <-> at c:/perl/site/lib/DateTime.pm line 1693."

the code i modified as below:

#declare packages used
use DateTime                   qw( );
use DateTime::Format::Strptime qw( );

my $format = DateTime::Format::Strptime->new(
    pattern => '%Y-%m-%d %H:%M:%S',
    time_zone => 'local',
);


while(<TEMP>)

{

       my @line=split(";");
       my @start_time1=($line[3]=~/(\d+)-(\d+)-(\d+)\s(\d+):(\d+):(\d+
+)/);
       my $start_time = $format->parse_datetime($line[3]);;           
+              #parsing start time
       my $stop_time  = $format->parse_datetime($line[4]);            
+              #parsing stop time
       $_->set_time_zone('local') for $start_time, $stop_time;
       print "$start_time, $stop_time\n";
       
       
       my $min_start = DateTime->new( year => 2007, month => 5, day =>
+ 1,hour=>0, minute=>0,second=>0, time_zone => 'local' );
       my $max_stop  = DateTime->new( year => 2008, month => 4, day =>
+ 30, hour=>23, minute=>59,second=>59, time_zone => 'local' );        
+  
      
       $start_time = $min_start if $start < $min_start;
       $stop_time  = $max_stop  if $stop >= $max_stop;
       
       my $dur = $start_time->delta_ms($stop_time);
       
       print"%d minutes and %d seconds\n, $dur->in_units(qw( minutes s
+econds ))";    
       
}
[download]

many thanks for your help !