Does StockTrim consider out of stock days historically?

Yes, if you were out of stock, StockTrim can ignore this from your historical demand that the forecasts are based on (it will base them on the historical sales trend).

Statistically the way the system calculates this is to ignore the 'zero days' in your sales history at times where you were out of stock, so the historical trend will be applied to your forecasts and not take these days into account.

If you have the order schedule enabled, you can see graphically how your expected stock on order and stock on hand levels will go in the future, and balance out to an optimal level.