A durable power of attorney is a power of attorney by which a principal designates another his attorney in fact in writing and the writing contains the words “This power of attorney shall not be affected by subsequent disability or incapacity of the principal, or lapse of time” or “This power of attorney shall become effective upon the disability or incapacity of the principal,” or similar words showing the intent of the principal that the authority conferred shall be exercisable notwithstanding the principal's subsequent disability or incapacity, and, unless it states a time of termination, notwithstanding the lapse of time since the execution of the instrument.