I found a solution here and modified it slightly for usage with Antville.

Add to the skin 'Javascript':


function dateDiff(createdate) {
datethen = new Date();
datenow = new Date();
diff = new Date();
datethentemp = new Date(String(createdate));
datethen.setTime(datethentemp.getTime());
datenowtemp = new Date(String(" [Macro not allowed in sandbox: now] "));
datenow.setTime(datenowtemp.getTime());
diff.setTime(Math.abs(datethen.getTime() - datenow.getTime()));
timediff = diff.getTime();
weeks = Math.floor(timediff / (1000 * 60 * 60 * 24 * 7));
timediff -= weeks * (1000 * 60 * 60 * 24 * 7);
days = Math.floor(timediff / (1000 * 60 * 60 * 24));
timediff -= days * (1000 * 60 * 60 * 24);
hours = Math.floor(timediff / (1000 * 60 * 60));
timediff -= hours * (1000 * 60 * 60);
mins = Math.floor(timediff / (1000 * 60));
timediff -= mins * (1000 * 60);
secs = Math.floor(timediff / 1000);
timediff -= secs * 1000;
document.write("Posted " + weeks + " weeks, " + days + " days, " + hours + " hours, " + mins + " minutes, and " + secs + " seconds ago");
}


-or- for a less complicated output:


function dateDiff(createdate) {
datethen = new Date();
datenow = new Date();
diff = new Date();
datethentemp = new Date(String(createdate));
datethen.setTime(datethentemp.getTime());
datenowtemp = new Date(String(" [Macro not allowed in sandbox: now] "));
datenow.setTime(datenowtemp.getTime());
diff.setTime(Math.abs(datethen.getTime() - datenow.getTime()));
timediff = diff.getTime();
weeks = Math.floor(timediff / (1000 * 60 * 60 * 24 * 7));
timediff -= weeks * (1000 * 60 * 60 * 24 * 7);
days = Math.floor(timediff / (1000 * 60 * 60 * 24));
days = weeks * 7 + days;
document.write("Posted " + days + " days ago");
}

Then replace the story.createtime-Macro in the skin 'recently modified item' (or 'Story display') by this code:

<script type="text/javascript">
<!--
dateDiff(" [Macro not allowed in sandbox: story.createtime] ");
// -->
</script>


Voila.