Try this. Create a snippet called DaysOld and paste in the following code:
<?php
/* DaysOld snippet */
if (empty($oldChunk) || empty($newChunk)) {
return "Can't find the chunks";
}
$days=isset($days)?$days:3;
$date2=$modx->documentObject['createdon'];
$date1=time();
$dateDiff = $date1 - $date2;
$daysOld = floor($dateDiff/(60*60*24));
return $modx->getChunk($daysOld <= $days? $newChunk : $oldChunk);
?>
Create two chunks called OldChunk and NewChunk with what you want to display (the names are case-sensitive so be careful to get them right).
Where you want the chunk content to appear, put this snippet tag:
[!DaysOld? &oldChunk=`OldChunk` &newChunk=`NewChunk` !]
It defaults to 3 days, but you can send a &days parameter to change that. It rounds down when calculating the days. If you want to round up, use ceil() instead of floor() in the snippet.