Workflow code to verify that a defect is being assigned to a cycle that is still within valid dates

I need to add some workflow code to do a basic validation but I have not been able to find any way to accomplish it and no other posts with suggestions.

Use Case:  I have set up Release A.  Inside Release A I have Cycle A and Cycle B.  Cycle A's dates are from June 1, 2013 to June 30, 2013 and Cycle B's dates are from July 1, 2013 to July 31, 2013.

When a defect is being created and a Cycle is selected I need to validate that the cycle selected is still valid based on the dates.

I can enter a look when BG_TARGET_RCYC is modified.  I can get the Name and NodeID of the Cycle that was selected.  But I can't figure out if the current date is beteween the start and end dates of the selected Cycle.

Anyone ever seen anything like this or know how I can use the Cycle_Fields data for the specific cycle that has been selected?

Thanks

Craig