And during an episode of his very informal Instagram Live chat show, "Current Mood," on Nov. 17, John sang a new opening verse to the hit single that he came up with on the spot after first taking a playful dig at Taylor.
"Love the song. I really like the song," John told his guest, Shawn Mendes, who duets with Taylor on a new remix of "Lover" that debuted on Nov. 13.
But, John added, "I get a little chuckle because she says [in the first verse], 'We can keep the Christmas lights up till January.' And I go, 'You're insane. Everyone keeps their Christmas lights up till January.'"
John kept up the good-natured ribbing, adding, "and I go, 'Can you really. 'Cause normally, I take my Christmas lights down on Dec. 29.'" (The actual Taylor-penned lyrics are: "We could leave the Christmas lights up till January / This is our place, we make the rules."
As Shawn and John chatted, the Canadian singer revealed that he was in Australia when he recorded his vocals for the new version of the song and not in the same studio as the American pop star.
But after some more joking around, John still wasn't ready to let go of the "January" lyric and soon started singing a cappella to the camera: "We can keep the Christmas lights up till January / And then about January 5th, we'll take the lights down / And then we'll put 'em in a box and we'll label that box 'Christmas lights' / Yeah, we'll put 'em in the attic till next December."
Shawn could be seen smiling in the same frame in the video, which was captured by a fan account. "That's a crazy love, I'll tell you," John quipped of the track, drawing laughs.
John and Taylor famously sang together on the song "Half of My Heart" on his "Battle Studies" album, which came out a decade ago in 2009. That's the same year he and Taylor reportedly hooked up and dated — when he was 32 and she was still a teenager. She all but confirmed it with her 2010 song "Dear John," which featured lyrics like, "Don't you think I was too young to be messed with?"