This copy of the Install macOS Mojave.app application is damaged, and can‘t be used to install macOS.

The error message is misleading, what it should say is the App Store has detected a previous download

Install macOS Mojave.app in your Applications folder.

When you go to the App Store I bet it says 'Open'

Go to your Applications folder locate Install macOS Mojave.app

move it to the Trash, empty the Trash.

Connect your mac to your router via cable, Restart your mac.

Go back to the Mojave download page it should now say Get or Download.

Do not use your mac while it is downloading or any other devices that share the same internet connection.

I suggest you look at my earlier post here https://discussions.apple.com/thread/251686879

You can either do it via the direct download off the App Store or use my preferred method of making a USB installer using DiskMaker X using the appropriate links from here https://diskmakerx.com/download/

I would normally agree with you but as you will see from the post someone else had the same issue and resetting the Date & Time fixes that problem. I personally installed Mojave on a Mac yesterday without running into the problem but that doesn't mean to say that no-one else is running into problems with it.

Yeah that is an overreaction. No need for you to reset your Date & Time to download an OS that has a

valid certificate.

That is why it took so long, I told you to connect via cable.