Insert parent item in proxy model

I would like to turn a flat treemodel into one with a tree.

In my case I have a list of items from a calendar and would like to put them in a tree with some general dates like

item1
item2
item3

to

today

item1

item2

tomorrow

item3

Is that possible within a qsortfilterproxymodel? And if - how?

Thanks, Gerd