You've got the function in place, but it's not comparing it to anything. That's why you're getting an error. Assuming you want exactly 30 days ago, you can do this;

SELECT...

FROM...

WHERE NewsTimeBroadCast.DateTimeSystem = dateadd('d',-30,getdate())

But if you want all the entries from the last 30 days, do something like this:

SELECT...

FROM...

WHERE NewsTimeBroadCast.DateTimeSystem > dateadd('d',-30,getdate())

Where possible, avoid running functions against columns because that will prevent index use and lead to scans.