Revision Diff Field

This module allows one field to track changes over a given time span in another field based on revisions. For example, if field A records the number of kittens you own, and field B is set to find the difference of field A over a 1 month span, field B would show how many new kittens you acquired in the past month. The difference is calculated whenever the entity is saved.

Currently, only nodes seem to have revisions, so it only works with nodes. So far this only does integer differences. If you'd like it to do something else, open a feature request issue.