I had to load an older OS X system onto one of our Macs at work but I couldn’t for the life of me get it to install. In this case it was OS X Mavericks but this solution is applicable for Yosemite or other versions as well I belive.
I had made an official USB installation disk with the app downloaded from the App Store and following the instructions from apple to make the drive: Create a bootable installer for OS X. So everything should be as genuine as possible, right?
When I started the install and right after selecting the hard drive to install to it started complaining. The message may look something like: This copy of the Install OS X Yosemite (or Mavericks, Mountain Lion, etc) application cant be verified. It may have been corrupted or tampered with during downloading.
The reason for this is certificate issues at Apples end. To get the install going again you just have to change the system date. In the OS X Installer find Utilities > Terminal. In the terminal window type: date 0201010116 and press Return. This will change the date on your mac to a date that allows the install to continue.
When the install is done, you can change the date back to what it should be in system preferences.
Hope this helps, I really scratched my head and wasted several hours before I got this to work again.