The impending debut of Windows 10 near the end of next month will start the clock ticking toward a deadline in 2016 when Microsoft will stop selling Windows 8.1 at retail, and a 2017 shut-off of sales of new devices armed with the soon-to-be-forgotten OS.

According to a long-established policy called "end of sales," Microsoft stops shipping an operating system to retailers one year after the launch of the OS's successor, and mandates that OEMs (original equipment manufacturers) stop pre-installing the older edition two years after the successor's release.

Those deadlines would signal an end to retail sales in 2016 -- likely in October, a month Microsoft favors for the stoppages -- and a close to new Windows 8.1 devices in 2017.

Microsoft's policy is more a guideline than anything: Retailers typically continue to sell a blocked edition of Windows from their stockpiles for years. And Microsoft itself has bent the rules. It has not yet called an end to OEMs selling Windows 7 Professional -- the business-grade SKU (stock-selling unit) -- even though by past practice that should have been halted in October 2014.

Instead, Microsoft went through a dance of first posting an Oct. 30, 2014, end-of-sales date for Windows 7, then retracted it, then restored it but only for consumer systems equipped with Windows 7 Home Premium. At the same time, Microsoft left the date blank for the halt of Windows 7 Professional installed on new PCs, saying only it was "not yet established."

That placeholder remained today, with the additional caveat that Microsoft will give customers and OEMs a one-year heads-up before it shuts down Windows 7 Professional on new devices. In other words, Microsoft will let computer makers factory-install the OS at least through June 18, 2016.