
I would set the work items to fixed work, make sure to enter work first when building out a project, and also attend a Q&A forum to better understand how work and duration related to the date fields (http://www.clarizen.com/resources/online-demo.html). Duration is always the difference between start date and due date, so likely changing the dates is modifying the duration, and in a fixed duration environment--which is the default--this will change what you have in the work column.