Let's start with some clarification of common names. Butterfly WEED is usually used for various Asclepias species. Asclepias tuberosa is orange and is commonly known as Butterfly Weed.

The plant in your picture is not Asclepias. It is one of the many forms of Argyranthemum frutescens. There is a cultivar called 'Butterfly' which looks like yours. I think you and the nursery guy were at cross purposes in your discussion.

Furthermore, people refer to many plants as 'Butterfly flowers' just meaning that they are attractive to butterflies. It's a description rather than a name.

So what all this illustrates, yet again, is that common names lead to no end of misunderstanding and it is important to try to ascertain the botanical name when obtaining new plants.

Argyranthemum frutescens will flower all summer long but it is not winter hardy. However, cuttings are easy and you can overwinter plants indoors much as you might keep pelargoniums or fuchsias.