7-Zip CMD: Add current date to archive and include only the last modified folder in archive

7z a -r "%DATE:~7,2%.%DATE:~4,2%.%DATE:~-4% Backup".7z

Will create the archive with DD.MM.YYYY Backup.7z Format.

Explanation: Echoing %DATE% prints the date in your regional date format setting.

D:\>echo %DATE%
Thu 11/04/2010

By using ~x,y specifier your are doing an in string/substring extraction of the string - where x is the starting character and y number of characters you wish to extract.

On your second point:

I only want to add last modified folder to archive(no matter how many folders exist in directory I just need the latest).

7z u -r "%DATE:~2,2%.%DATE:~5,2%.%DATE:~-4% Backup".7z 

should do it.