[PHP 8.x] Deprecated functionality: strtotime(): Passing null to parameter #1 ($datetime) (OpenMage#4339)

Author: addison74

app/code/core/Mage/CatalogRule/Model/Resource/Rule.php

@@ -183,8 +183,8 @@ public function insertRuleData(Mage_CatalogRule_Model_Rule $rule, array $website
 
         $customerGroupIds = $rule->getCustomerGroupIds();
 
-        $fromTime = (int) Mage::getModel('core/date')->gmtTimestamp(strtotime($rule->getFromDate()));
-        $toTime = (int) Mage::getModel('core/date')->gmtTimestamp(strtotime($rule->getToDate()));
+        $fromTime = (int) Mage::getModel('core/date')->gmtTimestamp(strtotime((string) $rule->getFromDate()));
+        $toTime = (int) Mage::getModel('core/date')->gmtTimestamp(strtotime((string) $rule->getToDate()));
         $toTime = $toTime ? ($toTime + self::SECONDS_IN_DAY - 1) : 0;
 
         $timestamp = time();