10.12 and 10.13 installers will now report the installer is "damaged" when launched. The actual problem is that their security certificate expired around Oct 22 of this month. (both of them, at the exact same time....?) So the fix is, as usual, to set the clock on the computer back to sometime before Oct 22, 2019 and that will "fix" the installer.

It seems no matter how many times I see this, it always takes me awhile to realize what's going on. Meanwhile I hunt for other problems, re-download installers, etc, and just generally waste my time trying to fix the reported error instead of the actual problem.

This time though it was doubly annoying because I was trying to upgrade a 10.11 computer. Since the high sierra installer wouldn't run, I went and tried the sierra installer, thinking I had to run that first, which failed in exactly the same way. This caused me to do a good deal of hardware testing, downloading, and file copying to try to "fix" the installer.

Thanks, Apple! Yes, I get it, it's a good idea to expire certificates as computing power nerfs encryption, but you could at least tell me what the problem is, rather than displaying a very misleading error message!

(and don't forget, combo updaters can have the exact same problem - I haven't tested them yet)

Not that I expect to ever use either of those installers again, but thanks for the heads-up.

And if I"ve ever got to use my archived Yosemite or El Cap installers I'll have to use some arbitrarily earlier date than 10-22-2019.

Alternatively, as reported by Jon in TIP: Do you need an older OS?, you can d/l updated versions of Yosemite, El Cap, and Sierra.

So those installers have been signed with newer (non-expired) signatures?

I've d/l'ed and launched all three.