Microsoft Corp. (MSFT) had big ambitions for its Windows 10 operating system when it launched it last summer, boasting that by the middle of 2018 1 billion devices will be running the latest OS.

But Microsoft has come to realize that is goal is unachievable, prompting it to move back its target.

Target Moved Back

In a statement, Microsoft marketing Chief Yusuf Mehdi said that “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 said it needs two or more years to achieve the goal and that there are 350 million monthly active Window 10 devices. Even though it is falling short of its target, which skeptics didn’t think was achievable, the software giant noted that Windows 10 is still its fastest growing OS. (See also: MSFT to Start Charging Users For Windows Upgrade.)

PC Market Headwinds

Microsoft pointed to its mobile handset business as the main reason why Windows 10 won't meet forecasts. But Microsoft faces other headwinds. The biggest may the personal computer market, where many consumers would purchase the new OS when upgrading their desktop computer. PC sales have been slowing as more people move to mobiles devices. Consumers with existing PCs aren’t as motivated as in the past to upgrade with the latest OS. As of June 29, Microsoft’s free upgrade program for Windows 10 is slated to end, which could further pressure its ambitious goal. (See also: Did Microsoft Buy LinkedIn for the Cloud Business?)