Microsoft in two weeks will finally retire a 2015 version of Windows 10, marking 29 months of support for the untitled feature upgrade, nearly as long a stretch as the time between the releases of Windows 8 and Windows 10.

Windows 10 1511 - Microsoft labels its feature upgrades in a yymm format - will receive a final set of security patches on April 10.

The retirement date had originally been slated for Oct. 10, 2017, but last November Microsoft extended it an additional six months, albeit only for commercial customers. "To help some early enterprise adopters that are still finishing their transition to Windows as a service, we will be providing a supplemental servicing package for Windows 10, version 1511, for an additional six months, until April 2018," Michael Niehaus, at the time a director of product marketing for Windows, said in a 2017 post to a company blog.

Customers running Windows 10 Enterprise or Windows 10 Education were given the support reprieve; those operating lesser SKUs (stock-keeping units), including Windows 10 Home and Windows 10 Pro, had their support curtailed last October.

Earlier this year, Microsoft added six months of support to all versions of Enterprise and Education, raising the support roof from 18 months to 24 for not only 1511, but also for 1607, 1703 and 1709. The less expensive, less expansive, Home and Pro, however, retained the 18-month support timeline.

Also destined for an April 10 retirement party is Windows 10 1607, the mid-2016 feature upgrade that will receive its last security patches that day on Windows 10 Home and Windows 10 Pro. Meanwhile, version 1607 on Enterprise and Education will continue receiving fixes until Oct. 9.

Windows 10's bifurcated support timelines - 18 months for some SKUs, 24 months for others - complicate what had been an easy-to-understand practice of patching versions for a year and a half. Period.

Microsoft has tried to inform customers of the support due them by reminding them in January's and February's cumulative updates that patches end for Enterprise and Education, version 1511. "The additional servicing offer for Windows 10, version 1511 ends on April 10, 2018, and doesn't extend beyond this date. To continue receiving security and quality updates, Microsoft recommends updating to the latest version of Windows 10," the March documentation said.