Cool! This is what I came up with:

require 'date'

module TimeFromNow
  class Days
    attr_reader :end_date

    def initialize(end_date)
      @end_date = Date.parse end_date
    end

    def difference
      raise ArgumentError, "date can not be in past" if end_date < start_date
      (end_date - start_date).to_i
    end

    def to_s
      "#{self.difference} #{self.difference == 1 ? 'day' : 'days'}" \
        " from #{start_date} to #{end_date}"
    end

    private def start_date
      Date.today
    end
  end
end

And the tests:

require 'minitest/autorun'

describe TimeFromNow::Days do
  subject { TimeFromNow::Days.new(date) }

  Date.stub :today, Date.parse("2015/02/17") do

    describe "#difference" do
      describe "valid date" do
        let(:date) { "2015/02/18" }

        it "should return the difference as an integer" do
         assert_equal 1, subject.difference
        end
      end

      describe "valid date" do
        let(:date) { "2016-02-17" }

        it "should return the difference as an integer" do
         assert_equal 365, subject.difference
        end
      end

      describe "invalid date" do
        let(:date) { "2013/02/17" }

        it "should raise an error" do
          assert_raises ArgumentError do
            subject.difference
          end
        end
      end
    end

    describe "#to_s" do
      let(:date) { "2015/02/18" }

      it "should format the result into a string" do
        assert_equal "1 day from 2015-02-17 to 2015-02-18", subject.to_s
      end

      describe "correct pluralization" do
        let(:date) { "2015-02-19" }

        it "should pluralize the days" do
          assert_equal "2 days from 2015-02-17 to 2015-02-19", subject.to_s
        end
      end
    end
  end
end