Apple - This copy of the Install OS X El Capitan application can't be verified. It may have been corrupted or tampered with during downloading

Follow this tutorial. This applies to the reply from @Cazuma Nii Cavalcanti. In short once you are at the first install page go to tools in the nav bar and open the terminal, in the terminal type date MMDDHHmmYY replacing the letters as follows.

MM - 2 digit month  01 - 12
DD - 2 digit date   01 - 31
HH - 2 digit hour   01 - 24
mm - 2 digit minute 01 - 59
YY - 2 digit year   > 15

once that is done go through the install normally. I just tried it and it worked with a USB install of OS X (10.11 - El Capitan) and it worked like a charm!

If setting to correct date doesn't work. Set to a date just after the os release.

If you are confident in the integrity of your installation media, you can use the terminal to bypass this message completely. The below instructions are for installs performed via a bootable USB stick:

1. Open the terminal:
2. Enter: installer -pkg /Volumes/Mac\ OS\ X\ Install\ DVD/Packages/OSInstall.mpkg -target /Volumes/"XXX" where XXX is the name of the disk you're installing to.
3. Wait for the installation to say it's complete. You will not see any sort of progress display.

This allowed me to install El Capitan a couple of weeks ago. I didn't have any internet access at the time, and my Mac had a completely empty hard drive, so I was very happy that I could force the install to go through.

I suspect that this method could be adapted for live installations, but failing that, consider creating a bootable installer and then following the above instructions.

I ran into this problem. I had installed a brand new SSD; I downloaded the installer and created a bootable USB stick. Everything looked raring to go. Hours later...

The ways to resolve this process:

1. Try setting the date to today's date as documented EVERYWHERE. If the install still fails:

2. Use htiutil to verify the checksum. Assuming you have the El Capitan download in the Applications folder, you can verify the download with the following command:

   hdiutil verify /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/SharedSupport/InstallESD.dmg
   

3. When did you download the installer? Apparently my problem was that I had downloaded the installer a number of days ago and the certificates had expired. If setting to today's date does not work; and the htiutil command came back with "is VALID"; try setting the date to the date your downloaded the installer.... or possible a few days earlier. I did that, and the install completed with no problems....

Good luck!