By the way this is not a date time issue, date is correct on my computer.
I think many people think setting the date to the current date automatically fixes the issue, but this is not the case. You have to remember the date you installed "El Capitan OS X installer.app" into your Application folder. For me, it was around Sept 2015. Right now is March 2016. When I got this error while reinstalling El Capitan OS X, I set the date to today's date (i.e. date 03061225616) and it did not work because I installed "El Capitan OS X installer.app" around Sept 2015 and the digital signature/certificates for my installer had expired by now. Once I reset the date to Sept 2015 (i.e. date 0923020415), the installation worked without a problem.
If you can neither remember nor correctly guess the date you installed "El Capitan OS X installer.app" into your Application folder, then you would have to delete it and reinstall it into your Applications folder so that the digital signature/certficates for the installer is no longer expired.
Also, setting the date manually in the terminal (i.e. date MMDDhhmmYY) worked for me. I heard setting the date automatically in the terminal (i.e. ntpdate -u time.apple.com) works for some people, but not for me. It most likely worked for them because they recently installed/reinstalled "El Capitan OS X installer.app" into their Application folder