Calculate difference between 2 timestamps using javascript

I have to calculate the difference between 2 timestamps. Also can you please help me with conversion of a string into timestamp. Using plain javascript only. NO JQUERY.

Here's my function:

function clearInactiveSessions()
{
    alert("ok");
    <c:if test="${not empty pageScope.sessionView.sessionInfo}">
        var currentTime = new Date().getTime();
        alert("curr:"+currentTime);
        var difference=new Date();
        <c:forEach items="${pageScope.sessionView.sessionInfo}" var="inactiveSession">
            var lastAccessTime = ${inactiveSession.lastUpdate};
            difference.setTime(Maths.abs(currentTime.getTime()-lastAccessTime.getTime()));
            var timediff=diff.getTime();
            alert("timediff:"+timediff);
            var mins=Maths.floor(timediff/(1000*60*60*24*60));
            alert("mins:"+mins);
            if(mins<45)
                clearSession(${item.sessionID});
        </c:forEach>
    </c:if>
}

i am posting my own example try implement this in your code

function timeDifference(date1,date2) {
    var difference = date1.getTime() - date2.getTime();

    var daysDifference = Math.floor(difference/1000/60/60/24);
    difference -= daysDifference*1000*60*60*24

    var hoursDifference = Math.floor(difference/1000/60/60);
    difference -= hoursDifference*1000*60*60

    var minutesDifference = Math.floor(difference/1000/60);
    difference -= minutesDifference*1000*60

    var secondsDifference = Math.floor(difference/1000);

    console.log('difference = ' + 
      daysDifference + ' day/s ' + 
      hoursDifference + ' hour/s ' + 
      minutesDifference + ' minute/s ' + 
      secondsDifference + ' second/s ');
}