Time Clock based on GMT

Hi! (: I have the following code that displays the time based on the end-users computer time clock. How would I change it so that the end-user sees the greeting based on GMT time zone???

Code:

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<SCRIPT LANGUAGE="Javascript">
<!--
{
        helixtime = new Date()
        hrstime = helixtime.getHours()
        if (hrstime < 7){timemsg = "You're up <B>really</B> early! "}
        if (hrstime > 6 && hrstime <12){timemsg = "Good morning! "}
        if (hrstime > 11 && hrstime <18){timemsg = "Good afternoon! "}
        if (hrstime >17){timemsg = "Good evening! "}
        document.write(timemsg)}
// -->
</SCRIPT>

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