2020 UPDATE: it seems that the installers that Apple is providing now work without you playing games with the date on your Mac. Try downloading a fresh copy direct from Apple. Below: links to Apple’s pages, where you can download these installers yourself. Your old installer isn’t going to work without some funky moves but the new ones probably will. So throw away the old one, download the new one, and try again.
- Yosemite (macOS 10.10)
- El Capitan (macOS 10.11)
- Sierra (macOS 10.12)
- High Sierra (macOS 10.13)
- Mojave (macOS 10.14)
Clicking the High Sierra or Mojave link takes you to the Mac App Store. When you get there, if you see “Open” instead of “Get” it means you already have the installer. It’s probably the one that won’t work. You have to get rid of it, and install a new one. Find the bad one in the Applications folder, with a name like “Install macOS High Sierra” or “Install macOS Mojave,” and put it into the Trash. Quit the Mac App Store app, then try the link (from above) again. This time, the App Store button should say “Get.” Click that button to download the installer anew.
The installer will try to run after downloading, so be ready for that. If you’re not ready to install, quit the installer, and come back later. (Remember, 10.13 and 10.14 installers will be waiting for you in the Applications folder.) If that doesn’t work, read the rest of this article and see if we have an answer for you.
Wouldn’t hurt to restart after downloading one of these installers, and before actually running them. A restart never hurts.
Trying to install Mojave or High Sierra or Sierra on a Mac, and getting messages about the installer being damaged– but you know it used to work? My guess: your installer isn’t damaged. Try changing the date on your Mac to sometime in 2018– maybe your “damaged installer” will magically work. Just go to the System Preferences, then Date & Time, and uncheck the box that says “Set date and time automatically.” Then change the year to 2018. Don’t worry about the day and month, just change the year. Then try your Mojave or High Sierra or Sierra installer again. Odds are good that the installation works perfectly. So much for the “Damaged Installer” error message!
This works because the installer isn’t really damaged. The real problem is that installers have a “certificate,” which is dated, and without a current certificate the installer won’t run. This is usually a good thing, except when the certificate isn’t kept up to date. In this case, Apple dropped the ball, and these Apple installers (and a bunch of their other installers) quit working. So, we turn the clock back to when the certificates were still valid, and the installers run without a problem.
You can turn on the “Set date and time automatically” check box after the installation is finished. This is really important. If, after installing Mojave or High Sierra, you can’t get on the internet, check the date. You probably forgot to set it to the proper date after doing the installation. Make that correction and you are all set.