I am zone 5b. I never go past September 15 any more. Actually I pretty much try to call it done by the first week of September. But I have planted in late September and the plant survived fine. We have had lows in the 40's already. First frost is around the middle of October. So I would wait until late April early May next year.
If you do go ahead and divide do mulch very well for winter and they will probably be okay. Especially if you can disturb the roots as little as possible and replant as clumps.
