Hello, variables in iRules have three scopes: local, static, and global.
Your syntax would assume a local variable is set. If nowhere in your code there's a statement to create and set "DATE_DD" value, you'll get the error you're seeing.
Local variables are scoped to the connection, and are deleted when the connection is cleared.
The type of information you're trying to retrieve is available, you need proper syntax to call it, example:
set now [clock format [clock seconds] -format "%d/%b/%Y:%H:%M:%S %z"]
