my $last_id;
my $last_date;
my @times;

for (;;) {
   my $line = <DATA>;

   my ($id, $date, $time) = split(' ', $line||'');

   if ($. != 1) {
      if ((!defined($id) && @times)
      ||  $id   ne $last_id
      ||  $date ne $last_date
      ) {
         print(join("\t", $last_id, $last_date, join(',', @times)), "\n");
         $#times = -1;
      }
   }

   last unless defined($line);

   $last_id   = $id;
   $last_date = $date;
   push(@times, $time)
      if !@times || $times[-1] ne $time;
}

__DATA__
848	05/23/06	11:00
848	05/23/06	12:30
848	05/23/06	13:00
848	05/23/06	14:00
848	05/25/06	11:00
848	05/25/06	12:00
261	05/24/06	11:00
261	05/24/06	12:30
261	05/24/06	13:00
261	05/24/06	13:00
261	05/24/06	13:00
261	05/24/06	13:00