generate_dates_in_between.rb

def gen_dates(arr)
    arr.reduce([]) do |acc, v|
      if acc.last
        d = Date.parse(v)
        diff = (Date.parse(acc.last) - d).to_i

        if diff == 1
          acc.push(v)
        else
          acc.concat((1..(diff-1)).map { |i| (d + i).strftime("%Y-%m-%d") })
          acc.push(v)
        end
      else
        acc.push(v)
      end
    end
  end
  

# output
gen_dates(["2016-09-21", "2016-09-20", "2016-09-18", "2016-09-12"])
["2016-09-21", "2016-09-20", "2016-09-19", "2016-09-18", "2016-09-13", "2016-09-14", "2016-09-15", "2016-09-16", "2016-09-17", "2016-09-12"]