I haven't really been able to test it, but I think it should work (except for the part about GMT):
<?php
$day = date(D);
$gmt = date(O);
if ($day == "Sat" && $time >= 14 && $time <= 16) {
echo "This is the HTML to output if the time is between 2 and 4 pm on a saturday";
}
else {
echo "This is the HTML to output if its not...";
}
?>