Inject mutable state (time)

di_clock_spec.rb

describe '#call' do    
  let(:today)  { Date.parse('20/08/2018') }
  let(:clock)  { double('Clock', today: today) }

  it 'generates report up to last month' do
    subject = described_class.new(clock: clock)
    # ...
  end
end

class SendReport
  def initialize(dependencies = {})
    @clock = dependencies.fetch(:clock)   { Date }
  end
  
  def call
    # ...
  end
  
  private

  def today
    @clock.today
  end
end