Corrupted El Capitan installer?
You can try the following:

Go to System Preferences and manually change the date to January 1st 2016. More specifically, deselect Set date and time automatically and then manually enter the date and click Save.

After doing this, double click on the El Capitan installer and follow the prompts to install the software.

If you run into problems, a couple of additional things you can try are:


Disconnect from the internet before manually changing the date and stay disconnected until you've done the installation 
Copy the installer onto the hard drive rather than running it from the USB


Let me know how you go.