At the end of each month and prior to the final day of the month, the MarketPress Store Stats display two copies of the current month, rather than showing the current month and the previous month. I'm attaching screen shots. You'll see that my date/time settings in WordPress and on the server are New York time, set properly.
Obviously this isn't a tremendous problem, but I figured it's something to address if it's happening to other installs as well.
Thanks for your help. I'm opening support access if you need it.