Out of sheer curiosity, what would the due date be in February if it's normally on the 30th on every other day of the month, considering that the Credit CARD Act of 2009 requires it to be the same day each month?
The answer is simple: The credit card company would avoid having the due date be the late enough in the month make it an issue.
In fact the card company should set a due date so that if the normal due date that month is a Saturday, and Monday was a holiday, then on Tuesday it would still be the same month.
Over the long term the due data makes very little difference to the credit card company. In fact if you ask they will allow you to adjust the due date to fit your pay cycle. I did this when I went to an employer who paid us monthly. It was an easy request. I don't remember if they allowed me to pick virtually any day, or if they had a only some dates they allowed. Starting the next month I had a new due date.