What to consider when passing a date and times?
Timezone
The billwerk server time is CET / CEST. Regardless of the user language you chose in your profile or the local settings (timezone) of your browser, all the dates in our application are displayed in CET - Central European Time (Europe/Berlin timezone) from October to March and in CEST - Central European Summer Time from March to October, respectively to Daylight Saving Time (DST).
Time stamps format
The timestamp is stored in the backend in ISO 8601 format (YYYY-MM-DDTHH:MM:SS.sssssZ / Rest API 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.
Date picker behavior
When you set a date, the Europe/Berlin midnight time is considered. When the time is taken into account, it is also influenced by the DST. This applies for example to timestamps as Today and Now.
Datetime pickers adjust the time to the corresponding contract phase start:
Example: a contract includes a phase starting at 26.03 2:01 am CEST.
If you pick 27.03 in the datepicker, it will return to you 27.03 2:01 am CEST.
Even in case, you choose 25.03, it will display 25.03 2:01 am CET, even if the 25th of March is not in DST, which would mean only +1h offset from UTC instead of +2h.