js processing php output timestamp does not match the solution_javascript skills

I wonder why the timestamp passed from PHP to JS for processing is always mismatched. It turns out that the JS timestamp is 13 digits, including 3 digits of milliseconds, while PHP only has 10 digits that do not include milliseconds.

var nowtime = (new Date).getTime();/*当前时间戳*/ 
/*转换时间，计算差值*/ 
function comptime(beginTime,endTime){ 
var secondNum = parseInt((endTime-beginTime*1000)/1000);//计算时间戳差值 

if(secondNum>=0&&secondNum<60){ 
return secondNum+'秒前'; 
} 
else if (secondNum>=60&&secondNum<3600){ 
var nTime=parseInt(secondNum/60); 
return nTime+'分钟前'; 
} 
else if (secondNum>=3600&&secondNum<3600*24){ 
var nTime=parseInt(secondNum/3600); 
return nTime+'小时前'; 
} 
else{ 
var nTime = parseInt(secondNum/86400); 
return nTime+'天前'; 
} 
} 
t = comptime(timestamp,nowtime);//timestamp为PHP通过ajax回传的时间戳