In XPath, you can only retrieve MendixObject instances, simple attribute values or aggregated values. To find the weeks elapsed after a date, you can execute OQL queries with the function DATEPART in Java.
It is possible to use date part functions in XPath, but only in constraints, such as week-from-dateTime, see https://world.mendix.com/display/NRG/XPath+week-from-dateTime.
the solution is on this post. Thanks for everyone's help.
It sounds as if you want to retrieve other data than is in your database (weeks between is not in your DB). XPath is not for that, it's a filter. What do you want to accomplish?