Assuming that the attribute DepartureDate in entity DepartureDate has type DateTime.
Create a variable with name DepartureDate with the type DateTime with value [%BeginOfCurrentMonth%] (make sure you create the variable before the retrieve with XPath).
Then, your XPath should look like so:
[BaseStf.SpecialAttendance_PersonalInformation/BaseStf.PersonalInformation/DepartureDate > $DepartureDate]
Hello, I think you are looking for '[%BeginOfCurrentMonth%]' or '[%BeginOfCurrentMonthUTC%]'.
Also see section 3.3 of XPath Keywords & System Variables - Studio Pro 9 Guide | Mendix Documentation
Hope this helps!
Using token [%BeginOfCurrentMonth%] you can get the date and time at the beginning of the current month. And then you can parse this value to extract month field from it.