Returns an event part start (DayPilot.Date).
There are two cases when the visible event box does not represent the full event duration:
- The overnight events will be shown as two or more event parts (each in one day).
- If non-business hours are hidden (DayPilotCalendar.HeightSpec == HeightSpecEnum.BusinessHoursNonScroll) and the event starts or ends in the non-business hours.
In these cases partStart() can return different value than start().