This article originally appeared in Forbes.

Former Secretary of State Hillary Clinton has made efforts to appeal to working women a centerpiece of her presidential campaign. One issue she continues to bring up is equal pay and how she thinks the federal government should address the issue. In a speech on April 12, which is known as Equal Pay Day or the day that marks how much longer women supposedly have to work to catch up to what men earned in the previous year, Clinton hit on this theme:

“The last time I checked there’s no discount for being a woman. Groceries don’t cost us less, rent doesn’t cost us less. So why should we be paid less?”

She tweeted on April 15:

A typical woman working full-time is paid just 79% of what a man makes, but there’s no discount for being a woman.

We all want equal pay for women. The oft-repeated 79 cent statistic is used by liberals to justify more federal government regulation of employment pay decisions. But this statistic doesn’t take into account many important factors that determine pay as I’ve previously discussed. One such factor is hours worked. A new study out shows why this matters.

According to the 2015 American Time Use Survey by the U.S. Bureau of Labor Statistics, among full-time employees, men worked 8.2 hours compared to women working 7.8 hours. This might not seem like much, but it adds up.

And if you compare employed men and women (regardless of part-time work), employed men work an average of 42 minutes more per day than employed women. 42 minutes might not seem like much, but that is an extra 3.5 hours a week or 14 hours a month. I know I could get a lot more done with an extra day and a half a month of work.

It is fair to expect that whichever sex works longer hours would get paid more when you compare overall salaries. This study confirms that we need to account for hours worked in the discussion over equal pay.