Microsoft has set a date for a second Longhorn-themed Professional Developers Conference (PDC). The event will be held in September next year, almost two years after the company first detailed the Windows XP successor at the 2003 PDC.

Longhorn has undergone a lot of changes since that initial unveiling. Microsoft has sacrificed some key advances it had planned to make a 2006 ship date. And it now plans to offer updates for Windows XP and Server 2003 to support technologies previously reserved for Longhorn.

At the second Longhorn PDC, to be held between 13 and 16 September in Los Angeles, Microsoft should be able to give developers more concrete information on the operating system. A first Longhorn beta is scheduled to be available in the first half of 2005, several months before the event.

Microsoft has distributed several preview releases of Longhorn, including ones at the 2003 PDC and at the Windows Hardware Engineering Conference in May 2004. However, with all the changes, it could be risky for developers to start programming for the operating system, as their work might depend on features that get cut, one analyst said.

"The timing of this PDC will be better because we will get builds of the product that are more representative of what it is going to look like when it is done," said Michael Cherry, lead analyst with Directions on Microsoft. "I would like to know when the feature set gets locked down."

The PDC in 2003 was the first time Microsoft talked publicly about many of the features it planned for Longhorn. Bill Gates hyped the operating system as "the biggest release of this decade, the biggest since Windows 95."

In April, it said it was clipping some minor features in Longhorn in order to get a beta version out next year, but it said the product would still have all the major components it discussed at the 2003 PDC. In August, however, Microsoft said it had stripped Longhorn of the WinFS unified storage system, which Gates had at the PDC had called the "Holy Grail".

At the same time, it said it would offer updates to Windows XP and Windows Server 2003 to support the Avalon graphics system, as well as the Longhorn communications subsystem dubbed Indigo and the WinFX application programming model.

Last month, Microsoft released a community technology preview of Avalon, an incomplete test version for developers to play with. The preview is also meant to allow developers to experiment with Avalon on the current Microsoft operating systems.

Adding Longhorn functionality to older Windows versions should be one of the major topics at the PDC next year, Directions on Microsoft's Cherry said. "I am looking forward to the PDC to get some explanation about how I can write an application that takes full advantage of Longhorn, and then degrades gracefully when it is running on XP with Avalon, and then maybe even degrades more gracefully when running on just XP," he said.

For more info on where Microsoft is now with Longhorn read our feature: Microsoft goes to pieces.