--- a/vendor/magento/module-rule/Model/Condition/Product/AbstractProduct.php
+++ b/vendor/magento/module-rule/Model/Condition/Product/AbstractProduct.php
@@ -568,9 +568,9 @@
             foreach ($this->_entityAttributeValues[$model->getId()] as $value) {
                 $attr = $model->getResource()->getAttribute($attrCode);
                 if ($attr && $attr->getBackendType() == 'datetime') {
-                    $value = strtotime($value);
+                    $value = strtotime((string)$value);
                 } elseif ($attr && $attr->getFrontendInput() == 'multiselect') {
                     $value = ($value && strlen($value)) ? explode(',', $value) : [];
                 }

                 $model->setData($attrCode, $value);