Facebook overestimated how long people were watching videos on its platform by as much as 80 percent for two years, a new report from the Wall Street Journal says.

The tech company said in a post in its Advertiser Help Center a few weeks ago that a certain statistic had not included any views that were less than three seconds. That inflated the numbers to make it seem viewers were watching videos for longer than they were.

The disclosure has upset advertisers who purchase video time on Facebook in part around that metric, the Journal reports.

"Two years of reporting inflated performance numbers is unacceptable," the ad buying agency Publicis Media wrote in a memo to its clients.

The metric that was affected by Facebook's calculation is called “Average Duration of Video Viewed." Facebook is now replacing that statistic with a measure called "Average Watch Time."

“We recently discovered an error in the way we calculate one of our video metrics,” Facebook said in a statement. “This error has been fixed, it did not impact billing, and we have notified our partners both through our product dashboards and via sales and publisher outreach. We also renamed the metric to make it clearer what we measure. This metric is one of many our partners use to assess their video campaigns.”

Facebook business and marketing executive David Fischer published a post responding to the Journal's report on Friday morning.

"This isn’t just about this error," Fischer wrote. "This is about how seriously we take our partners’ commitment to our platform, and how their investments with us wholly depend on the transparency with which we communicate. We know we can’t have true partnerships with our clients unless we are upfront and honest with them, including when we make mistakes like this one. Our clients’ trust and belief in our metrics is essential to us and we have to earn that trust."

Video ads are a huge business on Facebook. Publicis, for example, purchased $77 billion in total ads in 2015, the Journal said.

More of that money may have gone to Facebook than the agency would have chosen had Facebook's metrics been accurately represented.

Facebook has pushed video in recent years in both its algorithms and through platforms like Facebook Live. A Facebook executive in June said the social network will be "all video" in five years.

The disclosure from Facebook also affects numbers surrounding editorial video from media companies and other publishers.