Your time zone decides which day a session lands on, when your morning briefing is written, and what "today" means everywhere in the app. It is under Settings → Time zone.
Get it wrong and the symptoms are confusing rather than obvious: sessions on the wrong day, a briefing that arrives at the wrong time, a week that looks shifted by one.
If you are travelling for more than a few days, tell your coach rather than only changing the setting. It can move your training around the trip — and if you have a camp or a race at the other end, it will plan for that instead of being surprised by it.
Where this lives in the app: Settings › Display