There are two methods to bypass this. We will cover both, but the is re-downloading a patched version or manually changing your system date.
By following this guide, you should be able to get a functioning and successfully restore or upgrade your older Mac. mac os x el capitan 10116 installer dmg download fixed
Believe it or not, Apple still hosts El Capitan on its servers, but the link is hidden. To get a that can be fixed: There are two methods to bypass this