tl;dr
The timezone Klar uses for a source's reporting (including the day/month grouping of daily and monthly metrics) is set per data source.
Find it under Store Configurator → Data Sources → [open the data source] → Details → Timezone Setting.
It's a dropdown (e.g. Europe/Berlin). Set it and click Save.
Where do I set it?
Go to Store Configurator → Data Sources.
Open the data source (e.g. your Shopify store).
In the Details section, find Timezone Setting — "Select which timezone this data source uses".
Choose the timezone from the dropdown and click Save.
The setting sits right next to the data source's Currency setting.
What does it affect?
The timezone determines how that data source's dates are interpreted — which is what drives the daily and monthly grouping in your reports. If a day's totals look shifted (e.g. an order landing on the "wrong" day), the data source timezone is the setting to check.
Good to know
It's per data source, so different sources can have different timezones. For consistent reporting, set them the same way across your sources.
After changing it, allow a refresh for the reports to reflect the new grouping.