Those days vs. These days


Could I understand in such a way for the following phrases as:

  1. those days = in those days = in the past. Is there any difference?
  2. these days = at present but ‘in these days’ doesn’t exist



In those days refers to the past in an almost nostalgic way. Often the expression is used without ‘in’.

These days, as you say, means at the present time or nowadays and is not usually used with the preposition ‘in’.