これに変更'06-17-2013'
する'06/17/2013'
と、期待どおりに動作します
<?php
date_default_timezone_set('Europe/London');
$date = '06/17/2013';
if (strtotime("now") > strtotime("+5 days", strtotime($date))) {
echo '5 days have gone by';
} else {
echo 'not yet';
}
?>
strtotime 関数では、スラッシュ / とハイフン - の使用には違いがあります。
Dates in the m/d/y or d-m-y formats are disambiguated by looking at the separator
between the various components: if the separator is a slash (/), then the American
m/d/y is assumed; whereas if the separator is a dash (-) or a dot (.), then the
European d-m-y format is assumed.