How to get Mojave onto a High Sierra Mac?

[MacBird]

All you need to do is turn your wifi off, then switch the year in 'date & time" to 2018.
Them it'll work.
Once Mojave is installed you obviously can turn internet back on, and if your mac is set to get date and time automatically, the correct date will come back. (Or you can correct the date manually)

Good advice, I even had to disable SIP while installing Mojave.

 

[Geepaw]

I was having a problem upgrading to Mojave from the installer that has been in my applications folder for a long long time. The installer simply would not work and I could not find a replacement Mojave installler in the app store. I followed the advice from pshufd and deleted the installinfo.plist and the so-called "broken" installer was magically repaired and Mojave was installed seamlessly and without issue. Many thanks to the everyone who contributed in this thread. I am currently enjoying the wonder that is Mojave.

It's possible that this is the Expired Certificate problem.

There is a support article at https://discussions.apple.com/thread/8561304 with directions that may fix it.

Step 1. Go to your applications folder.
Step 2. Find the installer for macOS Mojave.
Step 3. Right click on the installer and click "Show Package Contents".
Step 4. Click on folder named "Contents".
Step 5. Click on the folder named "SharedSupport".
Step 6. Delete the file named "InstallInfo.plist".
Step 7. Enter your administer password to confirm.
Step 8. Now open the installer.

This solution is also seen at an the article at https://osxdaily.com/2019/10/24/fix-install-macos-application-damaged-cant-be-used-error-mac/

 

[The Hammer]

I was having a problem upgrading to Mojave from the installer that has been in my applications folder for a long long time. The installer simply would not work and I could not find a replacement Mojave installler in the app store. I followed the advice from pshufd and deleted the installinfo.plist and the so-called "broken" installer was magically repaired and Mojave was installed seamlessly and without issue. Many thanks to the everyone who contributed in this thread. I am currently enjoying the wonder that is Mojave.

I printed the article for reference. But I have to admit I'm a little reluctant to mess with a working system during a pandemic lockdown. Help might be hard to come by if there's an issue.