Mark,
You can use the VisualStartTime and VisualEndTime properties of GanttChart. Here is the sample code,
void ganttControl_TemplateApplied(object sender, EventArgs e)
{
this.ganttControl.GanttChart.AnchorTime = this.ganttControl.ProjectStartDate;
this.ganttControl.GanttChart.ViewportTimeRangeChanged += GanttChart_ViewportTimeRangeChanged;
}
void GanttChart_ViewportTimeRangeChanged(object sender, EventArgs e)
{
DateTime visualStart = this.ganttControl.GanttChart.VisualStartTime;
DateTime visualEnd = this.ganttControl.GanttChart.VisualEndTime;
}