The SEC Edgar Filing Tracker is the official place to track official company filings. Let us take the example of Apple reports. You see a long list of filings and a Date with a rather exact hour of report upload (e.g.
2021-02-08 16:31:03). What is the time zone of this date stamp?
The time zone used by SEC filings is Eastern Time. Specifically, Eastern Standard Time (UTC-05:00) or Eastern Daylight Saving Time (UTC-04:00), whichever was in effect at the time of filing.