DayPilot Month doesn't have built-in duration bar support. However, you can add a custom bar easily using "active areas" (in the Pro version):
protected void DayPilotMonth1_BeforeEventRender(object sender, DayPilot.Web.Ui.Events.Month.BeforeEventRenderEventArgs e)
{
e.Areas.Add(new Area().Left(2).Top(2).Right(2).Height(5).BackColor("red").Visible());
}
This code will add a red bar to the top of the event. If you want to use variable width you can use an inner div with percentage:
protected void DayPilotMonth1_BeforeEventRender(object sender, DayPilot.Web.Ui.Events.Month.BeforeEventRenderEventArgs e)
{
e.Areas.Add(new Area().Left(2).Top(2).Right(2).Height(5).Html("<div style='background-color:red; position: absolute; left: 30%; width: 40%; top: 0px; bottom: 0px'></div>").Visible());
}
This will display a red bar from 30% to 70% inside the event.
See also:
https://doc.daypilot.org/month/event-active-areas/