One solution that could work, and would not require any custom development, is to create a shipping method for each shipping time area. To illustrate what I mean I've attached a map of the US showing the shipping time for UPS Ground from our office in Pennsylvania to the rest of the country. Using this map you would create 6 shipping methods and then assign the appropriate states to each method. Where it gets tricky is states that have two or three different shipping times in them. In a case like that you could use a range or just go with the longer ship time.

When you create a shipping method that is not a real-time rate you can name the method, so you could call it something like "Ground (2 days)".


When using the UPS, FedEx, or USPS real-time shipping methods the names of each method are automatically set, so you cannot do what I suggest above.

It sounds like you may need to do some development to make it work the way you want it to. Probably the easiest way to handle this is to do your customization on the checkout page rather than trying to modify the shipping code in the Core or other assemblies.