Could you please try to reproduce the issue in our online demos: http://demos.telerik.com/wpf/
You could use the Configurator
demo, where you could set the DayStartTime and DayEndTime properties.
If you are able to reproduce it there, please send us some more information, a step-by-step guidance how this can be observed, as I tried to reproduce it in a sample project, but could not succeed.
Looking forward to your reply.