Hey,
You could do it like this:
<?php
$Time = date("G:i A");
$newTime = substr($Time,0,2);
$Time = str_replace($newTime.":",$newTime-2.":",$Time);
?>
The second line gets the first two numbers of the (24-hour) time and the third line replaces those two numbers with again those two numbers but decremented by 2.