Updating a clock display with the time
Theres a lot of things here.

Disclaimer: 

Using time stuff in JavaScript native is very hard and doesn't guarantee the cross browser. Also there are 300 of implementations of the same thing, plz don't make a new well. Please use a library like http://momentjs.com

First of all, is a little hard to read so, I started, formatting the code:

    function GetClock(){
        var d = new Date().addHours(1);
        var nday= d.getDay(),
            nmonth= d.getMonth(),
            ndate= d.getDate(),
            nyear= d.getYear();

        if(nyear&lt;1000)
            nyear += 1900;

        var nhour= d.getHours(),
            nmin= d.getMinutes(),
            nsec= d.getSeconds(),
            ampm;

        if(nhour === 0) {
            ampm = " ";
            nhour = 12;
        } else if (nhour&lt;12) {
            amPmSymbol =  " ";
        } else if(nhour===12) {
            ampm= " •";
        } else if(nhour&gt;12) {
            ampm = " •";
            nhour-= 12;
        }

        if(nhour&lt;=9)
            nhour = "0" + nhour;

        if(nmin&lt;=9)
            nmin = "0" + nmin;

        if(nsec&lt;=9)
            nsec = "0" + nsec;

        document.getElementById('clockbox').innerHTML =
            tday[nday] + ", " +
            tmonth[nmonth] + " " +
            ndate+", " +
            nyear + " " +
            nhour + ":" +
            nmin + ":" +
            nsec + "&lt;span class='ampm'&gt;" + ampm + "&lt;/span&gt;";
    }


So, i can see some issues and repeated code on this.


This function doesn't return anything. So, why is called as Get?, I recommend use updateTime
I can split date format, from hour/time format and am/pm symbol (I guess).
also, I eliminate the unused/usefulness variables from the clock creation. Now all additional functions return strings, so, can be concatenate with the template.

function updateTime(){
    var aDate = new Date().addHours(1);
    var ndate = aDate.getDate(),
        nyear = aDate.getYear() ? ;

    if(nyear&lt;1000)
        nyear += 1900;

    document.getElementById('clockbox').innerHTML =
        tday[aDate.getDay()] + ", " +
        tmonth[aDate.getMonth()] + " " +
        ndate + ", " +
        nyear + " " + getTimeAsFormatString(aDate);
}

function getTimeAsFormatString(aDate) {
    var hour = aDate.getHours(),
        mins = aDate.getMinutes(),
        secs = aDate.getSeconds();

    if(hour === 0) {
        hour = 12;
    } else if(hour&gt;12) {
        hour-= 12;
    }

    hour = hour&lt;= 9  ? "0" + hour : hour;
    mins = mins&lt;= 9  ? "0" + mins : mins;
    secs = secs &lt;= 9  ? "0" + secs : secs;

    return hour + ":" + mins  + ":" + secs +  "&lt;span class='ampm'&gt;" + getAmPmSymbol(hour) + "&lt;/span&gt;";

}

function getAmPmSymbol(hour) {
    return (hour &gt;= 12) ? " •" : " ";
}