When a defendant is in the custody of the commissioner on the expiration date of a final or temporary order of observation or an order of commitment, or on the expiration date of the last order of retention, or on the date an order dismissing an indictment is served upon the commissioner, the director of the institution in which the defendant is confined may, pursuant to section 730.70 of the Criminal Procedure Law, retain him for care and treatment for a period of 30 days from such date. If the director determines that the defendant is so mentally ill or mentally defective as to require continued care and treatment in an institution, he may retain the defendant beyond such period only pursuant to the admission procedures set forth in the Mental Hygiene Law. If the defendant is sought to be retained on an involuntary basis, the certificates of two physicians or, in the case of retention at a school for the mentally retarded, one physician and one psychiatrist, shall be filed and an application to a court for an order authorizing retention shall be made before the expiration of the said 30-day period.