How to lock max sales date using edit loack expression

Hello All,

I need to grey out current month forecast by changing edit lock expression for particular series, I tried  giving 

Month (  Sales Date  )=6  and also using  Month (  Sales Date  ) <7 in edit lock expression , here 6 and 7 refers to months, but it didn't work. Can someone let me know how can this be done using edit lock expression. Is there any other way to grey out current month apart from max sales date update script.

Note: We cannot update max sales it has be previous month only.

Thanks,

Sharath Chander M