Usually this property value should not be set directly; it gets its value automatically based on the VisualTimeSpan and SelectedDateTime properties (the latter one has DateTime.Today as default) so as to be aligned at the beginning of a current week or current month's first week. Setting it to a value that is not correctly aligned relative to the current control UI (for example, if it is not set to the first day of the week for the Month view) can cause an inadequate UI look.

To change a current period of time safely you can use the SelectedDateTime property or the SetRelativeStartTime(double) method or the SetRelativeStartTimeCommand command.