Changing statement date without notice

I have had a RZMC, issued by HSBC, since 2007.  Every three months or so I charge something small then PIF before the statement date which has been on the 20th since . . . forever.  This week I used the card for a small purchase.  I immedietely went to my bank bill pay and sent the balance.  Today, I checked my RZMC account online and it is showing a statement balance.  The statement was cut on the 9th, the same days my payment was received.

 

Now, I think that CCC’s probably have the right to change statement dates anytime they want to but aren’t they required to notify the customer in advance?  The only communication I’ve had from them recently was that I was being upgraded to “Platinum” status, whatever that means.