Js processing PHP output timestamp not on the solution

The timestamp passed in by PHP for JS processing is always out of place. The original JS timestamp was 13 bits and contained 3 milliseconds, while PHP only had 10 bits and contained no milliseconds.

var nowtime = (new Date).getTime(); 
 
function comptime(beginTime,endTime){ 
var secondNum = parseInt((endTime-beginTime*1000)/1000);//Calculate the timestamp difference

if(secondNum>=0&&secondNum<60){ 
return secondNum+' Seconds before '; 
} 
else if (secondNum>=60&&secondNum<3600){ 
var nTime=parseInt(secondNum/60); 
return nTime+' Minutes ago '; 
} 
else if (secondNum>=3600&&secondNum<3600*24){ 
var nTime=parseInt(secondNum/3600); 
return nTime+' Hours before '; 
} 
else{ 
var nTime = parseInt(secondNum/86400); 
return nTime+' Days ago, '; 
} 
} 
t = comptime(timestamp,nowtime);//timestamp for PHP through ajax The time stamp returned