The solution is to temporarily adjust your Mac's system date to a time before the certificate expired—specifically, when El Capitan was still current.
Launch your web browser (Safari is highly recommended for compatibility with Apple's secure transport protocols).
But (released July 2016) was the true masterpiece. It was the final, polished, patched version of El Capitan.
The best and safest sources are Apple's official support pages and trusted, well-established platforms. Here are your most reliable options: Mac Os X El Capitan 10.11.6 Installer Dmg Download
Open the mounted volume and double-click the file. Follow the onscreen instructions of the installer wizard.
Turn off your Wi-Fi or unplug your internet ethernet cable (vital, otherwise your Mac will auto-sync with the real time).
Navigate to the Apple Support Knowledge Base for Legacy Downloads. The solution is to temporarily adjust your Mac's
Open (Navigate to Applications > Utilities > Terminal ). Copy and paste the following command into the window:
The official direct download for Mac OS X El Capitan 10.11.6
Boot from your USB installer or Recovery partition. It was the final, polished, patched version of El Capitan
Official Apple Support page (recommended first step)
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