“Now and Then”arrives like a gift from that glorious Abbey Road in the sky on Thursday — despite the fact that John Lennon and George Harrison died in 1980 and 2001, respectively, leaving surviving Beatles Paul McCartney and Ringo Starr to finish the long-buried track on the right side of the dirt.And if you think that it’s the result of some magical mystery trickery of AI defiling the sacred ground that is the Beatles back catalog, thankfully, you’d be wrong.While “the last Beatles song” certainly benefits from advancements in technology that allowed it to finally be released — five decades after Lennon wrote and recorded the demo at his Dakota residence on New York’s Central Park West in the ’70s — “Now and Then” is a real Beatles tune that feels like it somehow got stuck in some mop-topped time warp before the “Imagine” singer was murdered in 1980. It’s a nostalgic trip back to the Baby Boomer glory days of yesteryear.