Day Two (IMO) is more of a style than an era. I think the term applies equally to a modern Camaro as it did to the 1st gens. It's about not leaving it stock and making your own. If I owned a Harley dealership I'd call it Day Two HD. Bikes are the perfect platform waiting for Day Two personalization. Case in point, this is my 6.2L 2020 Silverado when I purchased it. Nice but stock, like everybody else. Same truck "Day Two". Lift, knobby tires, 3/4" spacers, cab lights, fender flares, cold air induction, chip, flow master exhaust, air over ride bags, electric retractable bed cover, bed carpet, gun safe, cut and polish. Now it's mine. When the tires wear down I plan on smaller diameter wheels (these are Emmitt's, I'm thinking 20 or 18") with a bit more offset (get rid of the spacers) and bigger tires. That will be Day Three.
Back in the late sixties my mother worked in a store called Tempo, I believe it was a midwest chain similar to Kmart. They sold aluminum wheels in their automotive section, I remember trying to talk my dad into buying a set for his Buick to no avail @ $22.00 each I thought it was a deal!