I just confirmed that two of my archived installer images (Yosemite and El Cap) I made before Feb 2016 can no longer be used to make USB OS X installers . I thought they became corrupt (which is a very rare circumstance as I keep immaculate archives .) Now I am beginning to think Apple is playing games with us . Should we make a new thread with this issue ?
There have been posts about it on here but for some reason they keep getting deleted or moved.
Here is what you have to do.
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In case anyone encounters this problem ("OS X Installers Downloaded Prior to February 14 No Longer Work After Certificate Update”), here’s the fix
(Cut and paste):
1. In the OS X Installer, choose Utilities > Terminal.
2. Enter date 0201010116 and press Return.
3. Quit Terminal and continue the install.
That Terminal command sets your system date to 1 February 2016 -- before the certificate's expiration -- so the installer can continue. Once you have completed the installation, visit System Preferences > Date & Time to reset the system date.