ElCapitan

Mike Wuerthele said:

ITGUYINSD said:

Mike Wuerthele said:

Samhain said:

Mike Wuerthele said:

BigDann said:

You know there is a very easy fix here!

The installer looks at its certificates and the systems date. So as long as the systems date is older than the expiring cert the installer won't be the wiser!

As an example I just installed Sierra on an old 2011 iMac. I altered the systems Date & Time setting to manual and then back dated it to Jan 2017. Restarted the system and then ran the OS installer USB thumb drive I've setup. Once done reset the Date and Time to automatic and its done!

Workarounds (like yours) are good to know in a pinch. The fix is to download the images, with the new certificate, because setting back a system clock can have unintended consequences.

Mike you said ‘ The fix is to download the images, with the new certificate,‘ sorry if I am being dense, but how do you do this now the installers are no longer available?

Most are available now, minus a few. They should all be available by tomorrow.

The key point, is if you have an install disk or update image that isn't Catalina that's older than about three weeks, you're going to need to replace it.

Probably a dumb question, but now that the App Store is different, I can't find all the old OS versions (El Cap, Yosemite, High Sierra, etc.).  Where do we get those?

Not a dumb question. The Medium post has most of them linked. Here they are:

Sierra: https://support.apple.com/en-us/HT208202
High Sierra: https://support.apple.com/en-us/HT208969
Mojave: https://support.apple.com/HT210190
The rest: https://support.apple.com/downloads

Further back than that, the best way is hitting the App Store from a Yosemite Mac and checking out the Purchased tab -- assuming that's where you got the OS from.