Production Prep Buffer

I am working on our MRP settings and have not been able to get this field to function properly. On a manufactured part I have set the Production Prep Buffer time to 10.

My understanding from the support docs is that this should have the effect of moving the start date forward by ten days if possible.

I have a demand on 5/18/12 for the 10 of the part. MRP runs and creates a job for the parts. It is scheduled to start 5/16/12 and finish 5/18/12 even with the production buffer. My understanding of the field is that it ought to generate a job with a start date of 5/2/12 and a finish date of 5/18/12 as it would calculate it Finish Date - Operation Time - Production Prep Buffer time.

Any idea why this would not be working.

And yes, I has selected "use production buffer time" on the MRP run.