RadGantt needs to considering "working time" when plotting. You have workDayStart, workDayEnd, workWeekStart, and workWeekEnd which appears at this point to only control UI. My suggestion is as follows: let's say I have a task that is 8 hours long in duration(does not appear that I can set the duration on a task, its readonly), I set the task StartTime to be Friday 1PM and EndTime to be Friday 9PM, hence 8 hour duration, I would expect the control to consume its own configuration(defined by params above) and know that from 5PM - 9PM are not working hours, nor is Sat or Sun and to paint the additional hours on Monday from 8AM - 12PM. This is how MSProject and other gantt controls operate. Essentially I don't want to give you an EndTime, I should be able to simply set the StartTime and provide a duration and you would consume the working hours configuration and figure out the EndTime to plot.