有很多答案可以解决你的问题
How can I make a DateTimePicker display an empty string?
或
Set Null value in DateTimePicker in C#

请参阅此处msdn

public void CreateMyDateTimePicker()
{
   // Create a new DateTimePicker control and initialize it.
   DateTimePicker dateTimePicker1 = new DateTimePicker();

   // Set the MinDate and MaxDate.
   dateTimePicker1.MinDate = new DateTime(1985, 6, 20);
   dateTimePicker1.MaxDate = DateTime.Today;

   // Set the CustomFormat string.
   dateTimePicker1.CustomFormat = "MMMM dd, yyyy - dddd";
   dateTimePicker1.Format = DateTimePickerFormat.Custom;

   // Show the CheckBox and display the control as an up-down control.
   dateTimePicker1.ShowCheckBox = true;
   dateTimePicker1.ShowUpDown = true;
}