Never reset your mac without having a copy off your operating system on a bootable USB,
Apples Recovery Servers have not been update with OS's that have valid certificates and this
is the problem you are probably having.
When you reset the date did you follow this method.
Connect your mac to your router via cable, not WiFi
Boot to your Recovery HD, click on Utilities in the menubar select Terminal.
Make sure WiFi is switched off, it can reset the date back to today.
Enter a new date, for example or just copy and paste
sudo date -u 011421002016
press Return
enter your password
press Return
If Terminal returns an error saying sudo : command not found, then try again without sudo.
Now try downloading the OS.
If this works then when the OS is installed and booted up you can Open System Preferences> Date & Time
and reset the time back to today.
Do you have access to another mac that you know is/ was capable of running El Capitan.
If so follow these instructions for downloading El Capitan and creating a bootable USB installer.
How to get old versions of macOS – Apple Support
Go to Download macOS and click on OS X El Capitan 10.11
this downloads InstallMacOSX.dmg to your Downloads folder.
The next section can only be done on a mac that is capable of running El Capitan,
a mac that came preinstalled with an OS later than El Capitan will refuse to do the next bit.
When downloaded open to InstallMacOSX.pkg, double-click on
that and an installation window will open, this does not install El Capitan
but converts the InstallMacOSX.pkg to the Install OS X El Capitan.app which
you will find in your Applications folder, it should be 6.2GBs in size.
(If the installation window asks which disk you want to install to, you must pick
the disk that you are booted to at the time. Not any internal or external disk that
you want to eventually install El Capitan on, that is for later.)
To start the installation of El Capitan double click on the Install OS X El Capitan.app.
Create a bootable USB installer disk using the Install OS X El Capitan.app in the Applications
folder and the createinstallmedia command in the Terminal app.
Read the instructions here,
How to create a bootable installer for macOS – Apple Support
Having made the bootable USB installer you're ready to erase and then install a fresh copy of El Capitan.
The USB should be plugged directly to your mac.
Restart the mac while pressing and holding the option/ alt key.
In minute or two you will see the Startup Manager, select the USB and press Return.
When booted to the USB you will see a Utilities panel, select Disk Utility and press Return.
In the left hand panel highlight the Disk not the indented Volume, click Erase.
Give the Disk a name.
Format: Mac OS Extended (Journaled)
Scheme: GUID Partition Map
Click Erase.
When Done Quit Disk Utility.
Click on Install OS X press Continue the installation process should start up
follow the prompts.
If you get your mac back running through the Recovery Servers download El Capitan as described above
and make a bootable USB installer and keep it safe and use that for any time you want to reset your
mac and reinstall the OS. This is easier and quicker than using the Apple Recovery Servers.
