On two different occassions I’ve had this problem where I attempted to install OS X El Capitan via USB flash drive onto an older MacBookPro and encountered this error: “This copy of the Install OS X El Capitan application can’t be verified. It may have been corrupted or tampered with during downloading.”
Normally it is due to your date and time being off, usually if your battery died and your date got reset back to 1970. However, even with the date set via terminal to the current date and time I still have the same problem. Eventually I found that if you set the date back to 2013 via terminal, so using the command “date 0301120013” has allowed El Capitan to continue installing. Good luck!