Q1 Oct-Dec, Q2 Jan-March, Q3 April-June, Q4 July-Sept
Their release date calendar may not coincide with a fiscal calendar. Just because the fiscal year ends in September does not mean their release date will be fiscal year Q2.
Additionally, unless you work for Nvidia and have privileged information, you don't know Nvidia will release a gaming Volta GPU in Q2. But if I were to bet, Volta will be released in March or May at one of Nvidia's events.