Upgrading HD… Downgrading OS
After upgrading my failing HD to a new SSD I had to upgrade to OS X El Capitan. For various reasons (none of which really mattered in the end) I thought I needed to downgrade to OS X Yosemite.
At first I got error messages that the installer was corrupted or broken (checked several times that that was NOT the case). Finally, I found that Apple dates the OS X releases, meaning that if you try to install OS 10.19 in the year 2016 (like I tried) it is outside of the temporal bounds set by Apple.
Workaround
There is an easy workaround though. Before trying to install the OS, open the terminal and change the system time manually:
date [mm][dd][hh][MM][yy]
Which is “Month” “Day” “Hour” “Minute” “Year” without any separation.
Just roll back your time to when the specific OS you are trying to install was released. In my case I did:
date 1010101014
Which results in my new system time as:
Fri 10 Oct 2014 10:10:00 MDT
Hopefully this will help someone at some point. As always, use man date for more info.