XML : How to perform date operations in Dataweave-Mulesoft

I have a logic to filter out student who joined before 30 days.I have date of joining and lastdate in an xml filed.I have to subtract the dates from these two fields using data weave.

      <School>  <joindate>2015-10-18T00:00:00.000-08:00</joindate>  <lastdate>2016-01-18</lastdate>  </School>