If you're exposing an API to (or otherwise sharing data with) third parties, they're unlikely to have standardized on your localish timezone, unless they happen to share it. If you're consuming a third party API (or otherwise consuming data from them), they're unlikely to have standardized on your localish timezone, unless they happen to share it.

If you live in a disconnected bubble, your localish timezone might be consistent enough. You probably don't live in a disconnected bubble. Your localish timezone is not consistent. It is not "Universal". It is not UTC.