Though the hashtag was trending on social media on Friday, Affleck took to Instagram on Saturday with a trio of throwbacks -- just not of himself.
The Justice Leaguestar shared some epic black-and-white pics of Damon that look to be from the '90s, which is when the Massachusetts natives were first starting to make their names in Hollywood. (You remember that little movie the two worked on together called Good Will Hunting, don't you?) There's even a shirtless snap in the bunch, where Damon is crossing his hands over his chest as he stares sultrily into the camera.
"I know this is a day late, but Matt isn't on Instagram so thought I'd help him out. #OldHeadshotDay," Affleck captioned the headshots.
Though Affleck didn't share any epic old photos of his own in honor of Old Headshot Day, his ex, Jennifer Garner, sure did.
"Why so serious, twenty two year old Jen? #oldheadshotday. #1994," the Alias alum -- who shares three kids, daughters Seraphina and Violet and son Samuel with Affleck -- captioned the black-and-white image on Instagram.
Tons of other celebs also gamely took part in the throwback phenomenon. Kristen Bell also didn't share hers to Instagram until Saturday, writing, "Better late than never. Swipe left for my attempt at side ponies, attitude, and a bit of croquet."