When Microsoft launched its latest version of Windows a year ago, they aimed to deliver the operating system to one billion devices by mid-2018.
But an official from the tech company has now acknowledged that they will miss the deadline.
Microsoft is still determined to hit the 1 billion downloads goal, but will take a little longer to achieve, Windows marketing chief Yusuf Mehdi said in a statement.
"Windows 10 is off to the hottest start in history with over 350 million monthly active devices, with record customer satisfaction and engagement," Mehdi said.
"We're pleased with our progress to date, but due to the focusing of our phone hardware business, it will take longer than FY18 for us to reach our goal of 1 billion monthly active devices."
Microsoft has not been able to make much progress in the world of smartphones, which in recent times has been dominated largely by Apple's iPhone and Android's Galaxy line. The $7.2 billion decision to take over Nokia's mobile phones unit was ultimately proven ineffective.
Microsoft also recently terminated almost 2000 mobile related jobs while booking a $950 million restructure charge.
The company has enjoyed much better reception in the market of computers, but lost some of its goodwill when users complained about having been forced to upgrade them with Windows 10.
Despite falling short, Microsoft still believe Windows 10 is the fastest growing product with record user engagement and satisfaction.