Hey there! 
@samnichols is correct! More precisely, we take the timezone from whatever browser initiated the first request to hit our servers. When you have a data trigger or backend workflow that is triggered from a backend workflow, that data trigger or backend workflow “inherits” the timezone of whatever triggered it. You can find more details on this forum post as well as details for what we do when an API workflow is triggered externally (e.g. not from Bubble)