When creating sales orders in your IMS, there are several dates you can set. For example, the Created Date, the Order Date, the Required Date, the Dispatch Date (sometimes called Fulfilment date), Delivery Date, Received Date, Completed Date.
From an order planning point of view, it's important to recognise the difference between Demand and Fulfilment. The effective demand date is the date your customer wanted the item, and this is the date that StockTrim uses in each item of sales history to base your forecast calculations from.
Generally speaking this is the "Order Date". If that's not available in your IMS it uses the "Created Date" instead. Not the Fulfilment, Delivery, or Received Dates - these would cause inaccuracies in your demand model if they were used, since it wouldn't take into account any delays in fulfilling your customers' orders that you'd want to avoid in the future.
Note: Unleashed users can also configure whether to use the Unleashed "Required" date (under Settings in StockTrim).