Top Redmond types think that the number will be hit, but it will "not be 2018 as originally projected".
A spokesRedmond said:
"Windows 10 is off to the hottest start in history with over 350m monthly active devices, with record customer satisfaction and engagement. 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. In the year ahead, we are excited about usage growth coming from commercial deployments and new devices -- and increasing customer delight with Windows."
Microsoft Windows and Devices chief Terry Myerson made the original claim at Build 2015, noting the 1 billion would encompass all kinds of devices that would run Windows 10 in some variant, including desktops, PCs, laptops, tablets, Windows Phones, Xbox One gaming consoles, Surface Hub conferencing systems, HoloLens augmented reality glasses, and various Internet of Things (IoT) devices.
However it thought that the majority of those one billion devices would be PCs and tablets which was a market that dried up quicker than a pub in Utah during prohibition.
But Windows Phones running Windows 10 Mobile were also expected to help Microsoft reach that total by mid-2018 and that particular ship ran itself onto the iceburg and the spotty boy sank below the icy waters.
Microsoft has been aggressively trying to move existing Windows 7 and 8.x users to Windows 10 via a free upgrade programme that is ending on July 29.