Is it ok to keep on updating Date Time using Javascript's setInterval method?

Question

I am using Javascript's setInterval method to keep on updating the time of the Client computer. This is my code:

$(document).ready(function() {
    setInterval(function() { setCookieForDate(); }, 500);
});

function setCookieForDate() {
    var cookie_date = new Date();
    var serverDateTime = '<%=DateTime.Now%>';
    document.cookie = "LocalDate=" + cookie_date.toLocaleString() + "|" + serverDateTime;
}

Problem is that sometimes i still get stale (old) value.
when the value is being retrieved, it retrieves the old value even if when I check the cookie value it shows the UPDATED value.

Can any one suggest me any kind of modification in this code?