I like using
strtotime.
Try something like this:
CODE
<MTEntries lastn="1">
<?
$entrytime = strtotime("<$MTEntryDate format="%x %X"$>");
$now = strtotime("now");
$diff = ($now-$entrytime)/(3600); //because you want hours
if ($diff > 24) {
echo "It has been too long since my last post!!<br />";
}
?>
</MTEntries>
I always have to play with the time to make sure I'm dividing it by the correc thing.. $diff will give you the current time minus the last entry's time in seconds. And so since you want XX HOURS as the testing option, dividing it by 60 seconds * 60 minutes will give you the number of hours. Change 24 to be how many hours you want it to test (your XX value).
Let me know if this works for you; I only briefly tested it!

Kristine