In order to solve KOrganizer bug https://bugs.kde.org/show_bug.cgi?id=452306 (see below) the KDateComboBox => KDatePicker needs an API for setting the first day of the week, to override whatever is in the locale. STEPS TO REPRODUCE 1. Settings -> Time and Date -> Weekly start day: Monday (this has always been my setting.) 2. In agenda view, add an event. 3. Trigger the day picker by clicking the text box after "Start" or "End" OBSERVED RESULT The pop-up calendar should have Monday at the beginning of each row. EXPECTED RESULT The pop-up calendar has Sunday at the beginning of each row.