if date field is empty return null

DateTime cannot be null. Other than making it a string I know of two options.

First option, use the MinValue (or MaxValue) date as sentinel value. Then have checks for the MinValue where needed.

    return DateTime.MinValue; // set to MinValue
...
    // later in code check for MinValue
    if (Date != DateTime.MinValue)
    {
        // do something
    }

Option two, declare the DateTime as nullable. This will not be a DateTime object any longer and you will need to do type conversion when you map the nullable DateTime to a normal DateTime.

// the ? declares this as a nullable DateTime, e.g. DateTime?
public DateTime? Date2
        {
            get
            {
                if (calendarTextBox.Text == String.Empty)
                {
                    return null;
                }
                else
                {
                    return Convert.ToDateTime(calendarTextBox.Text);
                }
            }
            set
            {
                // you must then convert the DateTime? type to a DateTime type
                calendarTextBox.Text = Convert.ToDateTime(value).ToShortDateString();
            }
        }