Type date 071512002018 and press Enter. This rolls your Mac's internal clock back to July 15, 2018, a date when the High Sierra security certificates were fully valid. Quit Terminal and proceed with your macOS installation. Final Verdict

Before you install any downloaded file, you must check its integrity. You can do this by calculating its SHA-256 checksum to ensure it hasn't been tampered with. Open . Type shasum -a 256 (include a space at the end).

You can now delete the temporary file from your /tmp folder to free up space. Technical Specifications for macOS High Sierra 10.13.6

:Tools like Mist or gibMacOS can download the installer directly from Apple's software update servers to create a bootable DMG or ISO. Technical Specifications Download macOS High Sierra 10.13.6 Update - Apple Support

Because High Sierra is no longer signed by Apple for direct download (Apple removed it from the App Store in late 2019), many third-party sites host DMGs. To ensure , verify: