I am trying to calucate cycle times. I have 2 milestones (each with a
planned and actual date

Column A: Milestone 1 Planned
Column B: Milestone 1 Actual

Column C: Milestone 2 Planned
Column D: Milestone 2 Actual

If the period between the two periods has occured (Actual), then we want to
subtract D from B to determine the cycle time.

If they are blank then the cycle time is planned and the cycle time must be
calculated by

Column C- Column B (if it occured) or
Column C - Column A.

It would be great to distinguish the cycle and whether or not it is planned
or actual in one field. ALternatively, we can have a separate column for
planned (which would be blank if there was an actual) or actual column.