How does the 'days until replenishment due' calculation work?

Similar to the days left, but it's the days until your stock level will reach the reorder point.

This is the date when you should be due to place another supplier order. Not to be confused with the expected order arrival date.

Note that this takes into account items that you have on order - so it's possible to have very few days of stock left, but a replenishment due date further in the future.