class ActiveSupport::Duration::Scalar

def -(other)
  if Duration === other
    seconds   = value - other.parts[:seconds]
    new_parts = other.parts.map { |part, other_value| [part, -other_value] }.to_h
    new_parts = new_parts.merge(seconds: seconds)
    new_value = value - other.value
    Duration.new(new_value, new_parts)
  else
    calculate(:-, other)
  end
end