Unlike the question hour, the zero hour is not mentioned in the Rules of Procedure. Thus it is an informal device available to the members of the Parliament to raise matters without any prior notice. The zero hour starts immediately after the question hour and lasts until the agenda for the day (ie, regular business of the House) is taken up. In other words, the time gap between the question hour and the agenda is known as zero hour. It is an Indian innovation in the field of parliamentary procedures and has been in existence since 1962.

Source: Laxmikanth