Glencoe average start date would be mid January if you look back over the 60 years, latest start date was 19th Feb.
It should be noted however that Glencoe has historically had much later opening dates than Cairngorm/Glenshee. So while Glencoe would on average start in mid January the other resorts would start before Christmas.
I concure with ESD1711, this winter is what it is, and it is rubbish across the whole of Europe. I imagine it is pretty dire on the East coast in the USA too.