Last tested: Mar 18, 2020
The timezone that determines the timestamp that is appended to end of file names, is the system time zone. For Looker hosted instances, that will always be UTC, (just like how it is for logs and system activity)
All of the looker servers have UTC as the system time but some self-hosted servers are random timezones: https://docs.looker.com/admin-options/tutorials/timezones#system_time_zone
Self-hosted instances could have different timezones.