Create a datetime variable and name it OneYearAgo, set it to:
addYears([%BeginOfCurrentDay%], -1)
Then, set the xpath like this to find all objects with the date before the created date:
[StartDate < $OneYearAgo]
You can also use the system variable [%YearLength%]. For me the solution worked directly in the XPath constraint.
StartDate <'[%CurrentDateTime%] - [%YearLength%]']
Thanks @ruud, why didn’t I think of that!?