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The signatures expired early October of this year so any date before that usually works. Same for anything Mountain Lion- Mojave.

That's what I thought too.
But how come "date 080112002019" (Aug 1, 12:00:00 PDT 2019") works while "date 1010101015" (Oct 10 10:10:00 UTC 2015") doesn't?
 
I meant when installing 10.12 Sierra.
Both of the dates are before October 2019, but only the one you suggested works (not "1010101015" as suggested in the Diskmaker X FAQ).
 
I think you should always buy the latest one you can afford. It'll have the upper hand in performance, longevity and fixing any issues with the previous generation where possible. The Mini 5 is a huge leap ahead of the Mini 4 due to the length of time between models being released.
 
I meant when installing 10.12 Sierra.
Both of the dates are before October 2019, but only the one you suggested works (not "1010101015" as suggested in the Diskmaker X FAQ).

Just my guess (don’t take this as a fact) is that you cannot set the date before the OS was released. The certificate would probably throw an error that way. macOS sierra came out in 2016.
 
So it's a matter of trial and error (which is very time consuming -IIRC the error message appeared at the very end of a failed installation)? Or can the certificate effective/expiration dates for each OS be found somewhere?
 
We assume they were signed while it was released, and they all expired this October. Pretty easy to get in between those dates. August of 2019 should work for all of them.
 
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