Any time when the soil is warm and moist is a good time.
They ought to be long lived. Certainly the ones which survived in our last garden were there for the 20 years we lived there and my M.I.L. had Moorheim Beauty for even longer than that.
Have a look at www.specialperennials.com for lots of different ones and maybe some info.

I split my Sahin's Early Flowerer in the spring just as it is regrowing. I have had mine for about 15 years now, so a good genus usually.

Thanks, I thought I had read that they were short lived. So, that is good to hear.

Did yours flower the same year that you split them, seaburngirl?

Thanks, Owdboggy, I'll have a wee look at that.

yes they did and they are doing really well; been flowering since May.