Here is how I would do it.

- convert all times to seconds.
- make all times relative to the base time.
- determine a major key, the fifteen minute interval it's in relative to the base time.
- determine a minor key, the one minute interval it's in relative to the major key.
- save memory by processing each 15 minute interval as it completes, in the while loop.

Hope this gets you on track.

YuckFoo

#!/usr/bin/perl

  use strict;
  use DateTime;

  my $MAJOR_SIZE = 15 * 60;
  my $MINOR_NUM  = 15;
  my $MINOR_SIZE = $MAJOR_SIZE / $MINOR_NUM;
  my $BASETIME   = 0;

  my %ABBREVS;
  @ABBREVS{qw(Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec)} = (1..
+12);

  my ($bucket, $oldmajor);

  while (my $line = <DATA>) {
    chomp ($line);

    my (undef, $day, $mon, $year, $hour, $min, $sec) = split(/\W/, $li
+ne);

    my $time = DateTime->new(
      year   => $year,
      month  => $ABBREVS{$mon},
      day    => $day,
      hour   => $hour,
      minute => $min,
      second => $sec,
    );

    $time = $time->epoch();

    $BASETIME = $BASETIME || $time;

    my $relative = $time - $BASETIME;

    my $major = int($relative / $MAJOR_SIZE);
    my $minor = int(($relative - ($major * $MAJOR_SIZE)) / $MINOR_SIZE
+);

    if ($major != $oldmajor) {
      if (defined($bucket)) {
        process($bucket);
        $bucket = undef;
      }
    }

    if (!defined($bucket)) {
      $bucket = {};
      $bucket->{major}  = $major;
      $bucket->{minors} = [];
    }

    $bucket->{minors}[$minor]++;

    $oldmajor = $major;

    print "$line $time $relative $major $minor\n";
  }

  if (defined($bucket)) { process($bucket); }

#-----------------------------------------------------------
sub process {
  my ($bucket) = @_;

  my $major = ($bucket->{major} * $MAJOR_SIZE) + $BASETIME;
  print "\nmajor: $major\n";

  for my $i (0..$MINOR_NUM-1) {
    my $minor = ($i * $MINOR_SIZE) + $major;
    print "  minor: $minor $bucket->{minors}[$i]\n";
  }
  print "\n";
}

__DATA__
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