Example of clearing displayed value of DateTimePicker in WInForms with C#. Solution found at StackOverflow - http://stackoverflow.com/a/13364582/1816426

code to clear displated value.cs

dateTimePickerToClear.Format = DateTimePickerFormat.Custom;
dateTimePickerToClear.CustomFormat = " ";

DateTimeExampleForm.cs

using System;
using System.Windows.Forms;

namespace DateTimePickerExample
{
  public partial class DateTimeExampleForm : Form
  {
    DateTimePickerFormat defaultDateTimePickerFormat = DateTimePickerFormat.Long;

    public DateTimeExampleForm()
    {
      InitializeComponent();

      dateTimePickerToClear.Format = defaultDateTimePickerFormat;
      dateTimePickerToClear.CustomFormat = " ";
    }

    private void clearDateButton_Click(object sender, EventArgs e)
    {
      dateTimePickerToClear.Format = DateTimePickerFormat.Custom;
    }

    private void dateTimePickerToClear_ValueChanged(object sender, EventArgs e)
    {
      dateTimePickerToClear.Format = defaultDateTimePickerFormat;
    }

    private void searchButton_Click(object sender, EventArgs e)
    {
      // if .NET 4.0 is not available
      DateTime? startDate = string.IsNullOrEmpty(dateTimePickerToClear.Text.Trim()) 
                  ? null 
                  : (DateTime?)dateTimePickerToClear.Value;

      // if .NET 4.0 is available
      startDate = string.IsNullOrWhiteSpace(dateTimePickerToClear.Text)
                  ? null
                  : (DateTime?)dateTimePickerToClear.Value;

      // justo to check
      MessageBox.Show(startDate.ToString());
    }
  }
}