If you don't want to write the script then you can use a pre-existing one to make perform this calculation, like this script I've developed: Custom-made Adobe Scripts: Acrobat -- Calculate Time Differences in a Worksheet
These forums are for helping people with the use of their applications not doing their job or school homework.
This question has been asked several times. Have you used the "Search" feature to find similar post?
Use words or phrases like "number of days" or "days between".