How to install old OSX ( El Capitan )

1. download El capitan OSX

https://support.apple.com/en-us/HT206886

* execute Install dmg and install.pkcg file. and "Install El capitan.app" file  will be placed in /Applications/

2. create a bootable usb from "Install El capitan.app" 

https://support.apple.com/en-me/HT201372

3. Install OSX

if error message "This copy of the install OS X ... application can't be verified. It may have been corrupted or tampered with during downloading." occur during installing osx,
check your system date on your mac.




* checking date
Open Terminal and Type date

if date is not current time, change date.




* change date
type date month day time minute year

date 0201010120

2/1/2020, 01:01