Fender tag dates can be up to three weeks or so off, either early or late.
K-frame dates? I have not looked into it, I just do not go around comparing them.
For the guys into the date code thing, you should start a spreadsheet for any items you KNOW are original to the car. I need to do that myself for my T/A one of these days. So far, my engine assembly date is a few days after the date on the fender tag, so I know my car was made "late".