Creating Calculated Field in Business Intelligence to display how many days has passed from a specif

Summary

I need some pointers on how to create a calculated field in Business Intelligence. I would like a field that will tell me how many days it has been since a requisition has been unposted.

Content

I need some pointers on how to create a calculated field in Business Intelligence. I would like a field that will tell me how many days it has been since a requisition has been unposted. I located all of the fields I need, including the unposting date, but I am unable to create a field in the analysis in BI that will allow me to calculate the date from unpositing date to current. Does anyone have any pointers to help?