Lead time calculation is the calculation that takes place to calculate "Expected Receipt Date" from the "Order Date" or the other way around. Similarly, sales have the calculation to calculate "Planned Delivery Date" from "Shipment Date" or the other way around as well. Here, we will be focusing on the Purchase side but this will have same explanation for the Sales.
In any case, the interesting piece I would like to cover is how determines what calendar to consider when calculating lead times. As you know, there are many different calendars in NAV:
But, how all those relate when this calculation takes place? First of all, we need to have the following NAV premise in mind: if we don't have vendor calendar defined, NAV considers location calendar. Thus, if for whatever reason you have a location calendar while vendor does not have any (7 days per week), NAV will calculate the expected receipt date based on the location calendar. Here, most of the times, we wouldn't like this since we don't want the vendor lead time to be impacted by our location/warehouse calendar. If you are aware of this premise, you will understand better the design and you will have to setup a workaround: to create a vendor calendar with all days as working days (7 days per week). Having this in mind, NAV will find the vendor calendar (all working days) and will use this together with the vendor lead time.
Now, what are the different calculations that NAV is doing with lead times and calendars? a backward calculation and a forward calculation. Lets try to explain this a bit:
Here, the main thing is the fact about location calendar being used when vendor calendar is not set. As long as you are aware of this, you will anticipate issues by setting a 7 d/week calendar for the vendor to ensure this will be considered.