The billwerk server time is CET / CEST. This means that the time zone applicable in Germany determines which date and time is used for all operations.
The timestamp is stored in the backend in ISO 8601 format (YYYY-MM-DDTHH:MM:SS.sssssZ / https://developer.billwerk.io/docs/restApi/schema) and thus UTC. The Z at the end of the timestamp marks the time zone as UTC. A German date of e.g. 12.03.2021 00:00 (a.m.) is stored as 2021-03-11T23:00:00.0000000Z. Milliseconds are optional in the transfer.
Therefore, for operations that must be passed at 00:00 CET / CEST (e.g., due to a daily billing precision in the product family), YYYY-MM-DDT23:00:00.0000000Z must be passed via API in winter time and YYYY-MM-DDT22:00:00.0000000Z in summer time.
If a passed timestamp does not contain a time zone, undesired behavior may occur.
In the frontend, the display automatically converts from the stored timestamp to the local user/system time and, if an entry is made, the time is stored again in ISO 8601.