Yet if a same-sex couple had lived together for 18 years before being allowed to legally marry in 2015 and decides to divorce today, whether that union began two years or two decades ago is not legally clear.
"In some states, the courts will agree to tack on prior years of cohabitating … so it's viewed as a longer-term marriage than the legal date," Kauffman said.
That backdating is not a given, however. And if one spouse far out-earned the other, the stakes are high.
"The lower earner could be completely out of luck if they leave it to a judge," said Nancy Hetrick, a certified divorce financial analyst and senior advisor at Better Money Decisions in Phoenix, Arizona.
Even when one party tries to prove their relationship has lasted long beyond the legal start of the marriage — i.e., showing a joint bank account — there's no guarantee it will work.
"Not all states are going to accept or credit those arguments," Kauffman said. "Each state has their own statutory scheme for how they handle divorce. And the law of the state — and the case law interpreting it — is what will govern divorce."
The same complications arise in child custody issues. Often, whether a child was adopted or born to one of the parties, only one person in the relationship has legal parental rights even if both are raising the child. And when the couple goes to divorce, that lack of rights can stand in the way of the nonlegal parent continuing a relationship with the child.