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In order to change the "TimeMarkers" text of the ComboBox for the TimeMarkers all you need to do is in the EditAppointmentTemplate change the EmptyText
of the RadComboBox with x:Name="PART_TimeMarkers"
to the wanted value.
The easiest way of achieving this is by Implicit Styles. You can read this
help article that goes through on how to use Implicit Styles.
If you have any other questions feel free to write to us again.
the Telerik team