php 获取 sqlserver 内的datetime 格式的字符串，获取不到

[cyyjm]

<?php
header("Content-type:text/html;charset=utf-8");
date_default_timezone_set('UTC');
    $serverName = "172.16.200.12,1433";
    $connectionInfo =  array("UID"=>"uid","PWD"=>"pwd","Database"=>"china");
    $conn = sqlsrv_connect( $serverName, $connectionInfo);
    $yesday = date("Y-m-d",strtotime("-1 day"));
    $sql  =  "select * from tb_History2
where time BETWEEN DATEADD(mi, -6, GETDATE()-1) and DATEADD(mi, 6, GETDATE()-1)";
	$stmt  =  sqlsrv_query (  $conn ,  $sql  );
	if(  $stmt  ===  false ) {
	    die(  print_r (  sqlsrv_errors (),  true ) );
	}
    $arr = array();
    $n=0;
	while(  $row  =  sqlsrv_fetch_array (  $stmt ,  SQLSRV_FETCH_ASSOC ) ) {
	      
          $arr[$n] = array(
                           'id'=>$row [ 'id' ],
                           'value'=>$row [ 'value' ],
                           'time'=>$row [ 'time' ],      //这里有问题
                           'name'=>iconv('GB2312','UTF-8',$row [ 'name' ]),
                           's_id'=>$row [ 's_id' ],
                           'P_id'=>$row [ 'P_id' ]
          );
          $n++;
	}

    echo json_encode($arr); 
   	sqlsrv_free_stmt (  $stmt );
    sqlsrv_close( $conn);
?>
当我输出来的时候，time这个字段是空的，为什么会这样？