Sarah Michelle Gellar feels "lucky" to be married to Freddie Prinze Jr

Alongside some throwback photographs, Sarah - who has Charlotte Grace, 15, and Rocky James, 12, with Freddie - wrote on Instagram: "Happy Birthday @realfreddieprinze The world got very lucky the day you were born, but no one more so than me (and our children) I'm so glad I get to grow old with you...and that you will always have a head start!! (sic)"

Despite this, Sarah previously admitted that a successful marriage requires hard work.

The actress - who is best known for playing the lead role in 'Buffy the Vampire Slayer', the hit TV series - told Fox News Digital: "I think everything takes work in you, whether it's a friendship or a work relationship or a marriage. You have to put the work in.

"And we live in an extremely disposable society now. Your phone breaks, you don't fix it. You get a new one. I think that's a lot of the attitude toward relationships."

Prior to that, Freddie revealed that the loved-up couple try to keep their romance out of the spotlight.

The actor - who starred alongside Sarah in the 'Scooby-Doo' film franchise - said on the 'Oldish' podcast: "You can control it to a certain degree...

"I don't know how deliberate [others] are in trying to avoid that BS. I know how easy it's been for us because I don't go anywhere. I don't go to parties. People don't see Sarah and I out and about at the popular places."