When your customer is placing an order, in the sidebar, they will see an option that says "Requested Date" with a date bar underneath, where your customer will be able to request when they would like this order to be shipped.
On your side of things, as the administrator, you will be able to adjust the "Actual Date". Which is the date you expect to ship this order, this box will also automatically update after the order is shipped, if the shipped date doesn't match the date you selected and put in the order originally.
You are able to adjust what this date will automatically set to by adjusting your "Default Lead Time" in the order level settings. The "Default Lead Time" will adjust according to the date that the order is placed.