在Load中，初始化
this.dateTimePicker1.Format=DateTimePickerFormat.Custom;
his.dateTimePicker1.CustomFormat=" ";
在事件里写：
private void dateTimePicker1_ValueChanged(object sender, System.EventArgs e)
{
this.dateTimePicker1.Format=DateTimePickerFormat.Long;
this.dateTimePicker1.CustomFormat=null;
}
这样就实现了，在程序初始化时dateTimePicker显示为空