Windows 10 may be one operating system for all, but the many different versions aren't going to arrive all at the same time. Microsoft has said the PC compatible version of the OS will be ready first, and it'll launch before Windows 10 Mobile, the version for smartphones. According to Microsoft VP Joe Belfiore, the mobile edition isn't at the same development stage as the desktop and tablet version, hence the staggered release.
Speaking at an event held during the Microsoft Build conference, Belfiore explained the reason why mobile is lagging behind, saying, "We're adapting the phone experiences later than we're adding the PC experiences." The news gets worse, because he indicated the networks will have a hand in when Windows 10 is eventually released, and we know from experience they rarely move swiftly when it comes to updates.
This will also, presumably, affect Microsoft's plans to release a new flagship Windows smartphone. It has steadily sent low and mid-range devices out into the world since taking over Nokia's mobile division, but has so far ignored the more expensive hardware. If the plan was to launch a new flagship phone with Windows 10, we could be looking at much later in the year before one arrives.
Belfiore didn't provide even an approximate launch date for Windows 10 Mobile, saying only that the PC edition -- which is also suitable for large tablets -- will come during the summer. Potentially, we may be looking at a post-September announcement for Windows 10 on smartphones, perhaps coinciding in part with the IFA tech show in Berlin, which takes place between September 4 and September 9.
Windows 10 for phones isn't the only version of Microsoft's one operating system that will get its own release date. Versions built for smart devices associated with the Internet of Things, the Xbox One games console, and the HoloLens device won't be here during the summer either.