substract datetime fields by using fetchxml

hi everyone,

i use crm 2011 i am working on phonecall which is regarding with a custom entity that name is "new_custom"

i want to substract date from enddate(on new_custom entity) - system time and show the result(result should be day) string field on phonecall.

i dont know how to compare them by using fetchxml.

thx for answers.

Crm Software Specialist