ADDING DAYS TO #_EVENTDATES

Hi,
I’m trying to figure out how to create a placeholder that adds one week to the regular placeholder #_EVENTDATES.
I use this for creating recurring events where the recurrences themselves span over 2 days (the #_EVENTDATES and a week later). With that placeholder I don’t have to detach recurrences to create unique descriptions, but use the single event format settings to generate the dates.

I can’t make date-modifications to the ‘$this->output_dates())’ value.
my code (that doesn’t work):

case ‘#_EVENTDATESPLUSONEWEEK’:
$replace = strtotime(“+1 week”, $this->output_dates());
break;

Suggestions?
Thanks!