diff -u b/less.module b/less.module
--- b/less.module
+++ b/less.module
@@ -205,7 +205,7 @@
  * Helper function to create a new less dir.
  */
 function _less_new_dir() {
-  $less_dir = microtime(TRUE);
+  $less_dir = (string) microtime(TRUE);
   $less_path = file_directory_path() .'/less/' . $less_dir;
   file_check_directory($less_path);
   variable_set('less_dir', $less_dir);
@@ -216,8 +216,10 @@
  * Implements hook_cron().
  */
 function less_cron() {
+  $less_dir = variable_get('less_dir', '');
   foreach (new DirectoryIterator(file_directory_path() . '/less') as $file) {
-    if (preg_match('|^(\d+)\.\d+$|', $file->getFilename(), $matches) && time() - $matches[1] > 900) {
+    $filename = $file->getFilename();
+    if ($less_dir != $filename && preg_match('|^(\d+)\.\d+$|', $filename, $matches) && time() - $matches[1] > 900) {
       _less_recursive_delete($file->getPathName());
     }
   }