C#中图表横坐标日期设置问题

在C#的图表中我把横坐标改为日期格式，怎么设置横坐标的最大最小值为指定日期呢？

string montime = DateTime.Now.AddDays(Convert.ToDouble((0 - Convert.ToInt16(DateTime.Now.DayOfWeek)))).ToShortDateString();//本周的第一天
string fretime = DateTime.Now.AddDays(Convert.ToDouble((6 - Convert.ToInt16(DateTime.Now.DayOfWeek)))).ToShortDateString();//本周的最后一天
DateTimeSetting.datetime1 = Convert.ToDateTime(montime);
DateTimeSetting.datetime2 = Convert.ToDateTime(fretime);
thchart.ChartAreas[0].AxisX.Minimum = DateTimeSetting.datetime1.ToOADate();
thchart.ChartAreas[0].AxisX.Maximum = DateTimeSetting.datetime2.ToOADate();