Different ad based on UTM tags

We setup it like that:

<script>
function setCookie (name,value,hours) {
    var expires = "";
    if (hours) {
        var date = new Date();
        date.setTime(date.getTime() + (hours*60*60*1000));
        expires = "; expires=" + date.toUTCString();
    }
    document.cookie = name + "=" + (value || "")  + expires + "; path=/";
}
setCookie ('utm_source_session', 'cookie_value', 1)
</script>

But when someone came there later (f.e. second day) with UTM utm_source=bing.com, it shows him wrong ads (session will expire and will not recover after another access through these UTM).

Thank you.