Your indirectly talking about the year. "January" is for the current year and "last January" would be last year. I would interpret "this past" as most recent t

Your indirectly talking about the year. "January" is for the current year and "last January" would be last year. I would interpret "this past" as most recent thing from the current. So "this past January" would, to me, sound like the most recent January (Jan 2017)