Microsoft this week revealed that it has extended the mainstream support for Windows Server 2008 by an additional 18 months, from its original expiration on July 9, 2013 to January 15, 2015. However, unlike with Windows XP and its many support extensions, this change was required by Microsoft’s support lifecycle policies.

Microsoft revealed the change in its Support Lifecycle Lifeline newsletter. The news was first reported by my Windows Weekly cohost Mary Jo Foley.

“The Microsoft policy provides a minimum of five years of Mainstream Support or two years of Mainstream Support after the successor product ships, whichever is longer,” the newsletter explains. “Windows Server 2008 was originally scheduled to transition to Extended Support on July 9, 2013. Modifications to the expiration dates for Windows Server 2008 are a result of the launch of Windows Server 2012, giving customers the additional 2 years of support.”

In other words, because Server 2012 shipped this month, mainstream support for its predecessor, Server 2008, had to be extended to at least September 2014, which is beyond the originally scheduled July 2013 date, which (roughly) marks the five-year anniversary of the initial release of that product. I’m not clear on how they arrived at January 2015 (as opposed to September 2014).

If you’re wondering why Server 2008 R2 isn’t considered “the successor product” for Server 2008, Microsoft has consistently referred to its R2 products as optional, with the R2 bits essentially being delivered separately from the core Server version. So, in many ways, Server 2008 R2 is simply Server 2008 with some optional and additional features provided as add-ons.

Meanwhile, Windows XP, which has seen several support extensions, is finally moving toward its expiration date of April 8, 2014. When this milestone is reached, XP will have been supported for about 12 years, which seems like eons. But Microsoft’s support lifecycle policy now provides 10 years of support for business products (five years of mainstream support plus five years of extended support), with some period of “self-help” online support after that. And even Microsoft’s consumer and hardware products are supported for five or more years.