Today I put an SSD, 8GB of RAM and a new battery in my mid-2009 MBP. All components have installed succesfully. I've put the Yosemite Installer on an external HDD, and it boots up just fine on the fresh SSD, but once the installer hits '1 second remaining' it shows up with the error message 'This copy of the install OS X Yosemite application can't be verified. It may have been corrupted or tampered with during downloading'. I've changed the date/time via Terminal, but it's still coming up with this error every time I try.