The "in service" date, regarding the chassis, is the date the original purchaser bought the unit. That is the date Ford uses as the date to start their chassis warranty. FR also uses the original purchase date to start their one year warranty.
I'm "assuming" you have a Ford chassis. If you do go to the
Ford ETIS website and enter your VIN. You can down load a break down of your chassis information from when it was built. I don't know if Chevy has a similar website....