In order to install El Capitan from the Apple Recovery Servers it would be better to get your mac connected by cable rather than WiFi. I don't think Apple have bothered to update the certificates on their recovery software, so try changing the date in Terminal again, another reason for using a cabled connection is that WiFi has the habit of changing the date back to today.
So if your only alternative is to download El Capitan through the recovery servers, hitch your mac up to the internet via a cable.
Restart into your Recovery HD, make sure the WiFi is off, reset the date and time in Terminal.
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.
OR:
Do you still have an operating system on your mac, if so just boot to that and download El Capitan from here,
How to upgrade to OS X El Capitan – Apple Support (this has the updated certificates)
Go to Section 4 and click on Download OS X El Capitan, this downloads InstallMacOSX.dmg to your downloads folder.
When it has downloaded double-click on it and it will open to InstallMacOSX.pkg. Double-click on that and an installation window will open, this does not install El Capitan but uses InstallMacOSX.pkg to create Install OS X El Capitan.app in your Applications folder. Double-clicking on that will start the installation procedure.
OR:
If you can't use your mac to do this, then you could download El Capitan to another mac. This mac must be able to run El Capitan or have been able to run El Capitan at one time. Any mac that came preinstalled with an OS newer than El Capitan cannot do this. If you have or can borrow a mac that meets the requirements then you can download as mentioned above, follow the procedures to get Install OS X El Capitan.app in the Applications folder. Having done that plug in a USB stick (8GBs or larger) and create a bootable USB installer using the createinstallmedia command in Terminal.
How to create a bootable installer for macOS – Apple Support
Once you have created the bootable USB installer then you can plug that in to your original mac.
Restart the mac whilst pressing and holding the option/ alt key, in a couple of minutes the mac should boot to the Startup Manager, where you will see the bootable drives available, the USB should be on the right, select it and press Return.
The mac will now restart and boot to the bootable USB installer. Upon boot you will see a Utilities panel select Install OS X
and press Continue.