Hiding time slots in timeline View is not supported out-of-the-box by RadSchduler. Since this is a custom scenario, there are few drawbacks that come with it. For example, dragging and dropping appointments as well as creating appointment which start/end time is in the hidden slots, is not supported.
If you still need to implement this scenario, please take a look at our professional services: http://www.telerik.com/consulting/overview.aspx
the Telerik team
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