改进Model类的时间类型转换

library/think/Controller.php

@@ -137,7 +137,7 @@ class Controller
      * @param string|array $validate 验证器名或者验证规则数组
      * @param array $message 提示信息
      * @param mixed $callback 回调方法（闭包）
-     * @return void
+     * @return true|string|array
      */
     public function validate($data, $validate, $message = [], $callback = null)
     {

library/think/Model.php

@@ -874,7 +874,9 @@ abstract class Model implements \JsonSerializable, \ArrayAccess
                         $value = (bool) $value;
                         break;
                     case 'datetime':
-                        $value = strtotime($value);
+                        if (!is_numeric($value)) {
+                            $value = strtotime($value);
+                        }
                         break;
                     case 'object':
                         if (is_object($value)) {