Zero Hour in the Indian Parliament starts at

Zero Hour is an Indian innovation in parliamentary procedure that has existed since 1962 [c1][t6]. It is defined as the interregnum between the end of Question Hour and the beginning of the regular listed business of the House [t2]. Traditionally, Question Hour occupied the first hour of the sitting (11:00 a.m. to 12:00 noon), making 12:00 noon the starting point for Zero Hour [t2][t4]. This timing led to the nomenclature 'Zero Hour' as it coincides with the 12 o'clock hour [t4][t6]. While the Rajya Sabha shifted its Question Hour to 12:00 noon in 2014 (moving its Zero Hour submissions to 11:00 a.m.), the standard parliamentary definition and the Lok Sabha practice continue to associate the start of Zero Hour with 12:00 noon [t2][t5][t7]. It remains an informal device not mentioned in the Rules of Procedure [c1].

Sources

1. [1] Laxmikanth, M. Indian Polity. 7th ed., McGraw Hill. > Chapter 23: Parliament > ons > p. 242
2. [2] https://cms.rajyasabha.nic.in/UploadedFiles/Procedure/RajyaSabhaAtWork/English/639-649/CHAPTER19.pdf