use Benchmark;

$yours = '
$days = 10;
$then = localtime(time() + $days * 86400);';

$mine = '
$days = 10;
$then = localtime(time() + $days * 24*60*60 );';

timethese ( 100000, { 'yours' => $yours, 'mine' => $mine } );
__END__
Benchmark: timing 100000 iterations of mine, yours...
      mine: 30 wallclock secs (29.95 usr +  0.00 sys = 29.95 CPU) @ 3338.89/s (n=100000)
     yours: 30 wallclock secs (29.94 usr +  0.00 sys = 29.94 CPU) @ 3340.01/s (n=100000)