Also on Wednesday, Apple issued the first El Capitan GM, or "gold master," candidate to both registered developers and customers who have been previewing the new operating system through the public beta test program.
Apple typically runs through multiple GM candidates near the end of the process as it tries to stamp out as many bugs as possible before sending the software into the world.
Although no Apple executive mentioned El Capitan's pending release during the two-hour presentation Wednesday where they introduced new iPhone 6S and 6S Plus smartphones, the 12.9-in. iPad Pro and a revamped Apple TV, the company posted the Sept. 30 date on the El Capitan website.
Emails also went out yesterday to public beta testers with instructions on how to download the GM candidate from the Mac App Store.
Apple had a later start with El Capitan's developer preview program than it did in 2014 with Yosemite, but quickened the release pace and soon matched, then surpassed, its predecessor's cadence. After comparing the two tempos, Computerworld last week forecast a late-September launch for the upgrade. Yosemite appeared last year on Oct. 16.
El Capitan will run on the same Macs that now run Yosemite, 2013's Mavericks, 2012's Mountain Lion and 2011's Lion, according to Apple's system requirements. That means approximately 92% of the Macs that went online in August should be able to handle the upgrade.
If past trends repeat, El Capitan will capture a significant portion of the overall OS X share in its first full month of availability. The last two editions, Yosemite and Mavericks, had accumulated 37% and 32%, respectively, of all OS X user share by the end of their first full months.
El Capitan may not match those numbers, however. Its user share growth was more tepid during its preview stretch than Yosemite's, and at the end of last month stood at about two-thirds of Yosemite's at the same pre-release point. The relative lack of interest in El Capitan's previews may be due to its positioning by Apple as a performance, stability and reliability upgrade that includes few impressive new features.
OS X El Capitan will be a free upgrade, with the download posted on the Mac App Store Wednesday, Sept. 30.