Negative value for TotalCostOnDate that for specific date on --input will calculate the total cost of the inventory in the warehouse

CREATE OR ALTER PROCEDURE warehouse.TotalCostOnSpecificDate (@Date date)
AS
BEGIN

--declare @date date = '2013.04.01'
DECLARE @TotalInvertory dec (18,2)
DECLARE @TotalChange dec (18,2)

select @TotalChange = sum(it.Quantity* ih.LastCostPrice)
from Warehouse.StockItems i
INNER JOIN Warehouse.StockItemTransactions it ON it.StockItemID=i.StockItemID
inner join Warehouse.StockItemHoldings ih on ih.StockItemID = i.StockItemID
where it.TransactionOccurredWhen > @Date

select @TotalInvertory = sum(ih.QuantityOnHand* ih.LastCostPrice)
from Warehouse.StockItemHoldings ih
inner join Warehouse.StockItems si on si.StockItemID = ih.StockItemID

select @TotalInvertory - isnull(@TotalChange,0) as TotalCostOnDate
end

exec warehouse.TotalCostOnSpecificDate @Date ='2016-04-21'
exec warehouse.TotalCostOnSpecificDate @Date ='2015-01-01'

Start with the warehouse.TotalCostOnSpecificDate procedure shown in the issue and run it for the supplied dates against the relevant Warehouse tables. Trace the date filter and cost calculation, then verify that the returned TotalCostOnDate matches the expected inventory cost for those dates.