When creating calendar working times, the user has to select a working time template.
But one can forget this. The result is a worthless calendar where all days are in status "closed",
The result then is errors during MRP because MRP cannot generate planned production orders when the planning calendar mentioned in the parameters has no working days.
All this misery can be easily prevented by making the working time template a mandatory field.
Creating calendar working times without a template should be impossible.