There are actually two Purchasing Lead times in Dynamics NAV.
If we look at the Replenishment and Planning Tabs of a Dynamics NAV Item Card or SKU Card, we see Lead Time Calculation and Safety Lead Time. The Lead Time Calculation is what most people would call Purchasing Lead Time. The reason it is called Lead Time Calculation is that the user is allowed to enter a Dynamics NAV date calculation—for instance, 1M-1D or 5W or 35D.