Consider a context like the following:
There was a time when the United Kingdom and France were the world's foremost political powers, heading empires that spanned every continent. These two nations were at the forefront of the arts and technology. Th_se days are over. Now the United States and China share most of the economic influence, with Europe coming at best third when it manages to speak with a single voice.
Should it be these days, those days, or can either work? Do these and those give different impressions?
On the one hand, the days in question are in the distant past, which calls for “those”. It does seem a bit jarring to use “these days” to refer to the distant past, given that “these days” on its own means ”nowadays“. On the other hand, the days in question are the ones that have been mentioned just previously, which tends to call for this/these. They're also the first referent of two, making them the “this” and the present the “that”. Using “those days” is a bit jarring because it seems on the surface to refer to an unspecified or underspecified period on the past, rather than to the specific period that has just been mentioned. On the gripping hand, maybe it's just one of these cases where either this or that will do.
(Obligatory this is not a dictionary lookup request: I'm well aware of the differences between this/these and that/those, as well as the meaning of expressions such “these days” and “in those days”. My question is specifically about this case which does not involve a set expression and where there are arguments for both. I'm generally comfortable with choosing by feel but I originally wrote a paragraph like the one above with “th1se”, someone told me it should be “th2se” instead, and I'm now doubting.)
Best Answer
These days will soon be over (a future ending) is when you'd use these. You are still living through the days, but not for much longer. They will then become those days, separated from you by a distance of time.
(With a spatial separation, those can once again become these. With a separation in time, that is fundamentally impossible. This fact of finality colours all use of these days or those days. Not sure if that is necessary to say, but the implication may not be there in some other language I don't know of with different structure.)