Winter ends long before there are no more frosts. I'll guess our last significant frost tends to be in late April, but winter is over in March. Not because of the vernal equinox, but because the beginning of March is more like winter and the end of March is more like Spring. It can still snow after that, but the life expectancy of a late snow is pretty short after the sun comes out. But there's no doubt winter hangs on in NE Montana long after it's gone from central CT.