Michael Douglas and wife Catherine Zeta-Jones have once again proved they have the sweetest relationship. Douglas, 81, shared a post dedicated to his wife on their joint birthday on Thursday, September 25. "To my birthday sister-what a joy celebrating another year with you!" he captioned a photo with Zeta-Jones, 56, as they sat beside each other in a golf cart. "Congratulations on your spectacular game at the Celebrity Ryder Cup. Happy Birthday to the love of my life @catherinezetajones ❤️ 🎂 🎉." In the comments section, Zeta-Jones shared some kind words for her husband on their special day, writing, "Happy birthday darling❤️ I can't think of anyone else to share a birthday with.♥️." Michael Douglas and Catherine Zeta-Jones' Relationship Timeline Fans flooded the comments section with love for the couple on their birthdays. "Happy birthday to you both!🥳🥳Two extraordinary actors and the best couple in Hollywood!😉❤️," one person wrote, while another said, "Happy birthday to both of you! 🎉 Wishing you endless love, joy, and togetherness. May your journey continue to be filled with beautiful memories and happiness." The birthday wishes didn't end there, as Zeta-Jones took to her own Instagram page to share some cute pictures with her husband. In one photo, the twosome sat lovingly in a restaurant with smiles on their faces as they leaned close together. The Chicago actress also included a throwback photo of Douglas from his younger days, sporting brown hair. "Happy Birthday Michael! Sharing this special day with you is once in a lifetime!😘," she wrote alongside the photos. Arnold Jerocki/Getty Images Additionally, Zeta-Jones shared photos of her and Douglas packing on the PDA in her Instagram Stories. The Wednesday star wrapped her arms around Douglas' neck as he gave her a peck on the cheek in the adorable snapshots. Sharing a birthday and giving each other shout-outs every year is just one of the things fans have long admired about Douglas and Zeta-Jones' relationship. Upon their first meeting at the 1996 Deauville Film Festival, the Basic Instinct actor wasn't shy about sharing his interest in Zeta-Jones. "I said to her after about half an hour, 'You know, I'm going to be the father of your children.' It sounded good, and she said, 'You know, I've heard a lot about you and I've seen a lot about you, and I think it's time I say good night,'" he told Larry King in 2001. "[I] got her some flowers and an apology - anyway, so I guess it worked out OK!" Look Inside Michael Douglas, Catherine Zeta-Jones' $12 Million Home for Sale Douglas and Zeta-Jones got married in 2000, a few months after they welcomed son Dylan, now 25. Their daughter, Carys, now 22, arrived three years later. (Douglas also shares son Cameron, 46, with ex-wife Diandra Luker.) The lovebirds recently celebrated Carys' graduation from Brown University with a degree in film and international relations. "The night before graduation!!!! We are both such proud parents right now!! It's only just begun!! @carys.douglas 👩🎓🎓👩🎓," Zeta-Jones captioned a May 25 photo shared via Instagram of her and Douglas kissing their daughter on either cheek.