In drupal 8 there is easiest way to add the date format is simply go link for your website

"https://www.yourdomain.com/admin/config/regional/date-time"

OR

navigate to "Home  Administration  Configuration  Regional and language"

 

  • Now click on Add Format as shown above link
  • Add whatever name you like and Add Format string using php datetime using link
  • Example "l, F j, Y - H:i", "D, m/d/Y - H:i" etc.