简单：

DateTime start = dateTimePickerStart.Value.Date;
DateTime end = dateTimePickerEnd.Value.Date;
TimeSpan diff = end - start;
Console.WriteLine（diff.Days）;

i am count the day for differant 2 datetimepicker .once count this differant that differance store the textbox

Easy:

DateTime start = dateTimePickerStart.Value.Date;
DateTime end = dateTimePickerEnd.Value.Date;
TimeSpan diff = end - start;
Console.WriteLine(diff.Days);