In a microflow, you would create a variable that represents 15 days before today (set the value to addDays([%CurrentDateTime%],-15). Then use that variable in your Xpath on the retrieve.
Doing this purely with Xpath is difficult. You would need some way to get the date 15 days before today – you could use a Calculated attribute on a helper entity, but that is not the neatest solution IMO.
But it does not still work?
Mike meant it the other way around (I think, am no mindreader). Like so:
With FifteenDaysAgo being
Besides the microflow solutions, there is a way to do some basic date math directly in an XPath constraint. See this post for details: