From a customer standpoint you should be viewing your information in your purchase order and not in your supplier's sales order.
With that in mind I think you should use a confirmation at PO item level to record your initially expected date.
From a sales order perspective you still have a few dates at item schedule line details. In shipping you'll have:
Delivery date
Goods issue date
Loading date
Material avail date
Transportation plan.date.
Maybe one of them can be used.
I can get a lot more complicated so I'll stop here. Use PO confirmations or even just compare Sales order to PO.
regards,
Edgar