The result that you describe is by design. All intervals in the timeBar control are defined by a start and end date - the end date itself is excluded from the range. That is, all ranges in the timeBar control are interpreted as [start; end).
We already have the task for customization of the predefined interval types on our ToDo list. You can track the progress on the task and vote for it in our PITS system.
All the best,
the Telerik team