Oh! A fellow metric user, that's great! No conversions in my head.
Yes, I would wait a little bit until the temperatures warm up a little to start hardening your tree off. These trees are built to handle cold temperatures and dry spells but, that does not mean that you should overly stress your tree or.... brown needless. Not a good thing.
Anyway, with regard to winter; I would personally move your tree next to the house for some shelter during the winter. Window sills are actually really bad for plants, especially in winter.
I think that Scott (the HG) moves his trees into his garage during the winter.
Another option would be to move your tree directly into the basement.
I personally like the outside option but, that's just me.