you need to use the placeholder-tags, not the TV/Resource-field-tags:
[[+publishedon:strtotime:date=`%d/%m/%y`]]
The placeholder-tags are replaced by the values of the resource listed by getResources.
The TV/Resource-field-tags are replaced by the the values of the Resource which you actually have requested.
Born from sheer inexperience I had a problem today (v2.4.2) with getResources and showing the publish date. It seems [[*publishedon]] used on the article page itself produces a date string, while the [[+publishedon]] placeholder generated by getResources produces a unix timestamp. The apparent difference being that getResources is using strtotime before setting the placeholder.
The most obvious issue I've experienced is that on my page template I used:
[[*publishedon:strtotime:date=`%d/%m/%y`]]
But in my article summary chunk for the main blog page I had to shorten it to:
[[+publishedon:date=`%d/%m/%y`]]
...dropping ":strtotime" because it was no longer necessary. All good for efficiency I suppose, but I had to spend quite a bit of time trying to work out what was going on. Is this a known discrepancy or am I just misunderstanding something?