These are dark times for our country.
This is a dark time for our country.
Is there any difference in meaning between the two?
or is it OK to use either of them for the same meaning?
My feeling, not based on any research, is that "dark times" is idiomatic for the present and "a dark time" is idiomatic for the past.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.