Question about billing

This has bugged me for a while now.
 
I have the Sonar Platinum subscription. My subscription started on 1/21/15. Naturally, one would assume that the payment is due each month on the 21st. However, every month I've read the e-mail I receive when the payment is due and it tells me (for example, in August) "please make sure that your payment arrives at cleverbridge no later than 8/18/2015. Otherwise Cakewalk will deactivate your product."
 
My question is this: What is latest I can get my payment in without Cakewalk deactivating my product? If it's the 18th, the bill should say that the payment is due on the 18th, instead of the 21st.