`updated_at` and `created_at` are not UTC-based

Hi!

I'm using this package, and `updated_at` and `created_at` are correctly converted in `birl.Time`, but they are not turned into UTC. Is it on purpose? I find it always simpler to manipulate every timestamp in UTC in backend, and manage the timezones when user interaction is needed (sending a notification, displaying the time, etc.)

Start by tracing how updated_at and created_at are converted into birl.Time in the Hex API decoders. Review the existing discussion and package behavior to determine the intended timezone contract; done means the behavior is explicitly decided and verified.

Written by the indexing model from the issue text.