The winter where I live tends to veer either side of freezing, although snow doesn't usually stay for long. Now, I've go no ideological attachment to bare feet and I think cold feet SUCK. It's the reason why shoes were invented. So, I don't go barefoot in winter. The problem with that, is that when spring comes I have problems with blisters the first few times I go for long barefoot walks. So, what I did this last winter was walk for between 10 and 20 minutes each day on a gravel path around the pond in my garden (it's a large pond, so a decent path). The time depended on the weather; 10 mins if the gravel was frozen or snowy. I've just been on my first barefoot hike of the year, along the coast for seven miles over grass, asphalt (including the eroded kind with lots of grit), gravel and smooth paving slabs. No blisters at all!
I live in PA and believe it or not I went barefoot all winter this year. I made a New Year's resolution to not wear shoes all year. So far my feet have been bare since JAN 1.
In cold winters i do what Vasili does, to maintain the toughness. It does work, as my first proper hike, an all terrain 8 miler in March, caused no problems.