Now that's advice any couple can all agree upon.

By Jillian Kramer
Updated June 29, 2017
older couple eating cake
Credit: Willowpix / Getty Images

You've heard lots of marriage advice: Communication is key. Have date nights. Keep your sex life alive. But one piece of advice you likely haven't heard before? Eat cake to stay together for a long, long time—like, we're talking about 80 years.

That's how long Donald and Vivian Hart, both 99, have been married, and the happy couple say the key to their long marriage has been chowing down on cake together.

The couple opened up to ABC News about their 80th anniversary—they got hitched in 1937—and even ate cake during the interview. (A chocolate and a vanilla slice can be spotted on Vivian Hart's plate.) But in addition to the tradition of sharing sweet treats together, the Harts have a few tips from which new and old couples can learn a thing or two.

From a sofa in the Michigan assisted-living facility in which the Harts live, Donald Hart serenaded his wife—something he's done since they were dating. During the interview and over their years together, he sings Bing Crosby's rendition of "Let Me Call You Sweetheart."

"I used to sing it quite a bit when we were going together," he says. "It was the song that came to mind on our anniversary." (Read the lyrics and you'll understand.)

If you don't exactly have a singing voice, however, you're in luck—the Harts have yet another tip you can rely on. "Make sure the relationship is honest," says Vivian Hart.

Laughter can also ensure a long life together, Donald Hart says. "Have fun with each other," he instructs, "but don't make jokes at the expense of the other." That's probably good advice, especially if you're too honest and make fun of your partner's singing ability.

Lastly, Vivian Hart adds, "be always willing to help and stay alert to your spouse. We have prayer every day together. ... We don't keep our mind on our troubles."

Perhaps with those simple tips, a long, loving marriage can be a piece of cake.