For wife Michelle Obama's 55th birthday, former President Barack Obama shared a photo of them early in their relationship and a sweet caption

See Barack Obama's Sweet Throwback Photo for Michelle's 55th Birthday: 'You’re One of a Kind'

For wife Michelle Obama‘s 55th birthday, former President Barack Obama wants to make one thing clear:

“I knew it way back then and I’m absolutely convinced of it today — you’re one of a kind,” he wrote Thursday alongside a throwback image of the two, decades younger, smiling for the camera with his arm slung over her shoulders.

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Michelle is closing the book on a very busy 54th year which saw the release of her blockbuster memoir, Becoming.

Within a month of its November release, it sold three million copies. The former first lady also embarked on an oft sold-out book tour.

“I’ve been so humbled by the response to the tour thus far and the overwhelming interest we’ve received from so many communities we weren’t able to visit this year,” she told PEOPLE last fall.

Becoming includes discussions of the Obamas’ marriage and Michelle’s fertility struggles and childhood in Chicago.

In an at-home interview with PEOPLE, she talked about the value of being so candid about her life — and about her and her husband seeking counseling for their relationship, revered by so many.

“Because we’re role models, it’s important for us to be honest and say, ‘If you’re in a marriage and there are times you want to leave, that’s normal,’ because I felt that way,” she said.

Image zoom From left: Barack and Michelle Obama Barack Obama/Instagram

“There were definitely times when I wished things were different,” she told PEOPLE, “but I don’t think I ever thought, ‘I’m just checking out of this.’ “

In Becoming, Michelle also opened up about a miscarriage she suffered two decades ago. She ultimately conceived daughters Malia and Sasha via in vitro fertilization (IVF).

“I felt like I failed because I didn’t know how common miscarriages were because we don’t talk about them,” Michelle told ABC’s Robin Roberts in November. “We sit in our own pain, thinking that somehow we’re broken.”

She and the president, 57, wed in 1992 having first met at a Chicago law firm where they both worked — with her as his adviser.

Then, eventually, they kissed.