diff --git a/core/lib/Drupal/Core/Flood/MemoryBackend.php b/core/lib/Drupal/Core/Flood/MemoryBackend.php
index f911a98..770581f 100644
--- a/core/lib/Drupal/Core/Flood/MemoryBackend.php
+++ b/core/lib/Drupal/Core/Flood/MemoryBackend.php
@@ -47,7 +47,7 @@ public function isAllowed($name, $threshold, $window = 3600, $identifier = NULL)
     if (!isset($identifier)) {
       $identifier = ip_address();
     }
-    $limit = REQUEST_TIME - $window;
+    $limit = microtime() - $window;
     $number = count(array_filter($this->events[$name][$identifier], function ($timestamp) use ($limit) {
       return $timestamp > $limit;
     }));
@@ -60,12 +60,12 @@ public function isAllowed($name, $threshold, $window = 3600, $identifier = NULL)
   public function garbageCollection() {
     foreach ($this->events as $name => $identifiers) {
       foreach ($this->events[$name] as $identifier => $timestamps) {
-        $expired = array_filter(array_keys($timestamps), function ($expiration) {
-          return $expiration < REQUEST_TIME;
+        // Filter by key (expiration) but preserve key => value  associations.
+        $this->events[$name][$identifier] = array_filter($timestamps, function () use (&$timestamps) {
+          $expiration = key($timestamps);
+          next($timestamps);
+          return $expiration > REQUEST_TIME;
         });
-        foreach($expired as $expiration) {
-          unset($this->events[$name][$identifier][$expiration]);
-        }
       }
     }
   }