Assign a new calender in Project for the web . Calculation of duration goes wrong

Hello and good morning!

We created our own calendar for our project.

See procedure here: https://learn.microsoft.com/en-us/project-for-the-web/create-and-apply-a-work-calendar

If we now assign this calendar to the project, the calculation of the duration is no longer correct (see screenshot).

Does anyone have an idea of what that could be?

thnx for helping.

Nick