I’ve seen lots of customer service initiatives that promise “delivery by ……..” and no matter how quick that may be, there is still uncertainty about when it will be delivered, and customers will be anxious.

By contrast, “we will deliver at 3pm on the 25th” is very specific, and so long as you do deliver at the nominated time, every time, even if it is a few days longer than then quickest possible, customers just love the certainty.