How to promise a date to be delivered on a sale order to your customer?

When a sales/commercial department creates a sales order, he wants to tell the customer when he will be delivered (goods will be sent to him at the latest).  It would be in handy to see this on the screen when creating the sales order.  

The customer lead time in the product is something you need to specify for yourself.  You should try to avoid e.g. that you need to buy this product and it has a purchase lead time of 8 days, when your customer lead time is 7 days.  This would mean that your sales order line has a date of today + 7 days, but you have e.g. 1 day security days and those 8 days of purchase lead time, which will put the order date for the purchase to today - 2, which would mean that your purchase order is de facto too late.

The question is how can we know when the product will be available for shipment when there is somewhat complicated logistics process or manufacturing involved for procurement.  What a normal user should be able to tell would be the delivery lead time from their warehouse to the customer, but it is hard to know how much time is needed until the product will be available.

** I wanted to post this as a comment to the other answer, but I can't tell how to do it.  It seems I can only comment on the original question.

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