by_star.rb

   # Examples:
    #   Post.by_day
    #   Post.by_day(Time.yesterday)
    #   Post.by_day("next tuesday")
    def by_day(time = Time.zone.now, options = {}, &block)
      time = parse(time)
      by_star(time.utc.beginning_of_day, time.utc.end_of_day, options, &block)
    end
    alias_method :today, :by_day
    
    # Examples:
    #   Post.yesterday
    #   # 2 days ago:
    #   Post.yesterday(Time.yesterday)
    #   # day before next tuesday
    #   Post.yesterday("next tuesday")
    def yesterday(time = Time.zone.now, options = {}, &block)
      time = parse(time)
      by_day(time.advance(:days => -1), options, &block)
    end
    
    # Examples:
    #   Post.tomorrow
    #   # 2 days from now:
    #   Post.tomorrow(Time.tomorrow)
    #   # day after next tuesday
    #   Post.tomorrow("next tuesday")
    def tomorrow(time = Time.zone.now, options = {}, &block)
      time = parse(time)
      by_day(time.advance(:days => 1), options, &block)
    end

by_star_spec.rb

  def find(*args)
    method = description_args.first.sub(' ', '_')
    Post.send(method, *args)
  end

  describe "today" do
    it "should show the post for tomorrow" do
      find("tomorrow").map(&:text).should include("Tomorrow's post")
    end
  end

  describe "yesterday" do
    it "should be able find yesterday, given a Date" do
      find(Date.today).map(&:text).should include("Yesterday's post")
    end
  end

  describe "tomorrow" do
    it "should be able find tomorrow, given a Date" do
      find(Date.today).map(&:text).should include("Tomorrow's post")
    end
  end

gistfile3.txt

Failures for by_star only occur (seemingly) in a positive timezone where the local time is in a day in the future from UTC.
So in +1000 (Brisbane) these three specs only fail BEFORE 10am. After 10am it's a sea of wonderful green.

..................................F..F..F...................................

1)
'Post today should show the post for tomorrow' FAILED
expected ["Today's post", "Today"] to include "Tomorrow's post"
./spec/by_star_spec.rb:197:

2)
'Post yesterday should be able find yesterday, given a Date' FAILED
expected ["Today's post", "Today"] to include "Yesterday's post"
./spec/by_star_spec.rb:213:

3)
'Post tomorrow should be able find tomorrow, given a Date' FAILED
expected [] to include "Tomorrow's post"
./spec/by_star_spec.rb:230:

Finished in 0.263454 seconds

76 examples, 3 failures