Last week’s IRS ruling on same-sex marriages received a lot of attention. Going forward, a same-sex married couple must file federal income tax returns as married, regardless of whether the state where they live recognizes their marriage. In addition, same-sex couples may—but don’t have to—file amended returns for some earlier years to recoup any extra taxes they paid because they had to file as individuals.

But most reports on the ruling have missed an important detail: the IRS will begin applying the new rule on September 16. That gives same-sex couples who haven’t yet filed their 2012 tax returns just a few more days to file as individuals if they choose to do so. For some, that could mean a much smaller tax bill.

At issue are marriage penalties, the additional income tax some couples have to pay because they must file their federal tax returns as married. Because tax parameters aren’t proportionately higher for couples than for singles—for example, higher tax brackets for joint filers apply at less than twice the income as for individuals and phase-in and phase-out ranges don’t start at double the income level—some couples pay more income tax than if they were single. That’s most likely to be the case if spouses have similar incomes.

For example, if both spouses earn $100,000, have no children, take the standard deduction, and have no other income, their 2012 tax bill filing jointly would be $38,319. If they could file individual returns, each would pay $18,731 in income tax. The total—$37,462—would be $857 less than what they’d pay as a couple. They pay a marriage penalty.

Of course, many couples pay less tax than they would if they were single and thus get a marriage bonus. If one spouse earned all of the hypothetical couple’s $200,000 income and the other earned nothing, filing jointly would save them nearly $9,000, a hefty marriage bonus.

It’s not always obvious whether a couple will pay more or less tax because they are married. You can get an idea of the difference between filing jointly or as individuals using the Tax Policy Center’s Marriage Bonus and Penalty Calculator. But one warning: the calculator cannot take account of every tax situation—you’ll have to complete and compare joint and single tax returns to find the exact bonus or penalty. Of course, the same rules will soon apply to both same-sex and heterosexual couples.

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For a few more days, however, same-sex couples who were legally married last December 31 and who haven’t yet filed their 2012 tax returns can avoid marriage penalties one last time by filing individually. If you’re in that situation, you could save a lot of money if you get your 2012 tax return in soon.

Disclaimer: the Tax Policy Center doesn’t give tax advice. If you have any doubts about how the new tax laws would affect you, consult a real tax advisor.