They do that each eve of the month's ending day but you'll still see any yesterday's and today's sales added into the total for the month. Nothing is lost by them doing this on the evening of the day before the 1st day of the new month (Sept.).
BTW: Today is Saturday, August 31st, 2013. Nook Press only does a 'sort of' month's end total, only a couple hours before midnight each month. Doesn't really hurt anything.
I have to apologize to you because the sales from the last two days of August did not in fact roll over to the final tally for the month and some authors are saying only some of theirs have done so...so far. I only had two sales the last two days of August -- one from the 30th and one from the 3st and as it stands today, Sept 1st, they aren't added into August yet but I figure they will be.
If not, I have record of exactly which two they were and I will be inquiring with Nook Press, as to where they went.