OS X could not be installed on your computer

I never asked you to restart your mac after changing the date. After changing the date and it is successful, quit Terminal and then click on Install OSX from the Utilities panel.

If you are still getting the error then it looks like Apple are stopping you as this method was working

a few weeks ago. Have look through the El Capitan community for other posts the same as yours.

Make sure your mac is connected to your router using a cabled connection rather than WiFi,

if you are using WiFi it may be resetting the date to today.

The certificates for several of Apples OS's expired in October 2019, Apple have been very poor at getting these updated on their Recovery Servers with valid certificates. This may be the problem you are experiencing. 

Try this workaround, we are going to set the time and date on your mac to a date previous to the expiry date of the certificates.

Boot to your Recovery HD, click on Utilities in the menubar select Terminal.

Enter a new date, for example or just copy and paste

sudo date -u 011421002018

press Return

enter your password

press Return

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.

Ok it's difficult to keep up with which OS's accept the sudo command or not, in this case just

enter

date -u 011421002018

press Return

See if it works now.