public function validateEDate($attribute, $params)
 {
     date_Default_timezone_set('Asia/Colombo');
     $dt = Date('Y-m-d h:i:s');
     if (strtotime($this->end_date_time) < strtotime($this->start_date_time)) {
         $this->addError($attribute, 'Invalid date');
     }
 }
<?php

use yii\helpers\Html;
use yii\helpers\ArrayHelper;
//use yii\widgets\ActiveForm;
use yii\bootstrap\ActiveForm;
use dosamigos\datetimepicker\DateTimePicker;
use backend\models\Item;
//use yii\bootstrap\ActiveForm;
/* @var $this yii\web\View */
/* @var $model frontend\models\CustomerOrder */
/* @var $form yii\widgets\ActiveForm */
date_Default_timezone_set('Asia/Colombo');
$dt = Date('Y-m-d h:i:s');
echo $dt;
?>


<div class="customer-order-form">

    <?php 
$form = ActiveForm::begin(['layout' => 'horizontal']);
?>
  
     
    <?php 
echo $form->field($model, 'customer_id')->textInput(['maxlength' => 10, 'style' => 'width:100px']);
?>
   <!-- <?php 
echo $order_id;
?>