The text can be customized using BeforeEventRender event handler. This event is fired once for each event in the DataSource. You can set custom HTML by changing e.InnerHTML property:
protected void DayPilotCalendar1_BeforeEventRender(object sender, BeforeEventRenderEventArgs e)
{
e.InnerHTML = e.Text + " " + e.Start.ToShortTimeString();
}