Microsoft didn’t talk much about Windows 10 Mobile at its big press event yesterday, aside from unveiling the first devices that will run it (Lumia 950, Lumia 950XL, and Lumia 550). The company did slip out a timeframe for when the new operating system would arrive for existing Windows Phone devices, via the official Lumia page on Facebook (as spotted by WinBeta).

Windows 10 Mobile is the latest version of the company’s mobile operating system. It runs universal Windows apps, meaning one app can run a variety of form factors, supports developers porting their existing Android and iOS apps with minimal modifications, and offers a new feature called Continuum that lets you connect an external display, mouse, and keyboard and use your phone like a PC. As with the Windows 10 upgrade for PCs, Microsoft is giving Windows Phone 8.1 users a year to upgrade free.

Here is the comment on Facebook that finally gives a timeframe for the rollout:

Windows 10 is available on PCs and tablets now, and while Lumia 950 and Lumia 950 XL ship with Windows 10 for phones, broad availability of Windows 10 on other devices, including mobile phones, will begin rolling out in December. For more information on Windows 10 for Lumia, please visit http://www.microsoft.com/en/mobile/windows10/. For other concerns, feel free to contact our Twitter support team @LumiaHelp or visit http://answers.microsoft.com/en-us/mobiledevices for assistance.

Update: We also reached out to confirm with Microsoft that the timeframe has been set. “Broad availability of Windows 10 on other devices, including mobile phones, will begin rolling out in December,” a Microsoft spokesperson told VentureBeat.

December will be just the start. Not all devices will be supported from the get-go, and carriers will, of course, slow down the process.

Microsoft is aiming to have 1 billion devices running Windows 10 “in two to three years,”¬†though that includes not just PCs, but smartphones, consoles, and other devices as well. While PCs are naturally Microsoft’s biggest strength, the company is trying once again to tackle smartphones, phablets, and tablets.

In April, Microsoft explained that Windows 10 will launch on PCs first, and will arrive for phones sometime later. The company’s other devices, including the Xbox One and Microsoft HoloLens, will get the Windows 10 treatment afterwards.

The release gap between PC and phone versions makes a lot of sense. Windows naturally has a much bigger user base on the PC than on the phone, so Microsoft wants to focus on getting users to upgrade there first. Test builds of Windows 10 Mobile are behind their PC counterparts in terms of features and stability — the main reason for this is simple: Windows 8 was released before Windows Phone 8, so the latter simply got a later start. Windows 10 Mobile also has to go through carrier testing, something Microsoft naturally has little control over in various parts of the world.

Update on December 17: Microsoft has delayed the release. “This November we introduced Windows 10 to phones including brand new features such as Continuum and Universal Windows Apps with the introduction of the Lumia 950 and 950 XL,” a Microsoft spokesperson told VentureBeat. “The Windows 10 Mobile upgrade will begin rolling out early next year to select existing Windows 8 and 8.1 phones.”

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