From investors point of view September is a quarter. It’s Sep to Dec, or Q4.
Assuming we’re going by IRS, that should be July to September. Which fiscal quarter it is depends on when the company was established.
For example, the fiscal year at my work is from July to June so July to September 2021 is actually Q1 of FY 2022 for us.
Iirc, Apple’s fiscal year is October to September.