This is just for grins, but a thought came to me over my morning tea. How does this fare?:

#! perl -slw
use strict;
use Data::Dump qw[ pp ];

sub dhm2int {
    my( $d, $h, $m ) = unpack '(A2)*', $_[0];
    return ( ( $d - 1 ) * 24 + $h ) * 60 + $m;
}

sub int2dhm {
    sprintf "%02d%02d%02d", int($_[0]/1440)+1, int($_[0]/60)%24, $_[0]
+%60;
}

my @data = sort{ $b->[2] <=> $a->[2] } map[ split ], <>;

my $tally = chr(255) x ( 31*24*60 );

for my $d ( @data ) {
    my $s = dhm2int( $d->[0] );
    my $e = dhm2int( $d->[1] );
    substr( $tally, $s, $e-$s+1, chr( $d->[2] ) x ( $e - $s + 1 ) );
}

my @res;
my $i = 0;
while( $i < length( $tally ) ) {
    ++$i until vec( $tally, $i, 8 ) != 255;
    my $val = vec( $tally, $i, 8 );
    my $start = $i++;
    ++$i while $i % 1440 and vec( $tally, $i, 8 ) == $val;
    my $end = $i - 1;
    push @res, [ int2dhm( $start ), int2dhm( $end ), $val ];
}

pp \@res;
[download]