I've done splits in August and Septemeber, but where you are, I'd stick with August as the cutoff. But you may have to feed a lot to get them up for the winter. July should be pretty safe. A great time for splits is just before the flow. Take all the open brood and adhereing bees and put them in the split. The parent hive will make twice as much honey, with no brood to care for, and the split will have a chance to build up on the flow. It's a win/win.