DateTime.AddDays what value to pass to generate an exception

I have a small utility class that generate a hash code with input data. What class does & its algorithm is not significant to the question but it looks like below:

public static class HashCodeHelper
{
    /// <summary>
    /// Return a hash code to be added to the link in download data.
    /// </summary>
    /// <param name="baseDateTime">Today's date</param>
    /// <param name="rrid">Profile ID</param>
    /// <param name="downloadCode">Download code to be used for hash code generation.</param>
    /// <returns>Final result is a string in this format "20140715385"</returns>
    public static string GenerateHashCodeForExportLink(DateTime baseDateTime, int rrid, string downloadCode)
    {
        var todayDate = baseDateTime;

        int expireInDays;

        if (!int.TryParse(ConfigurationManager.AppSettings["ExportLinkExpireInDays"], out expireInDays))
            expireInDays = 30; // If not specified in web.config file then use default value

        var expiryDate = todayDate.AddDays(expireInDays);

        var currentDay = todayDate.Day;

        var expiryMonth = expiryDate.Month;

        char nthChar = Convert.ToChar(downloadCode.Substring(expiryMonth - 1, 1));
        var asciiValue = (int)nthChar;
        var mod = (rrid % currentDay);

        var computedHash = (asciiValue * expiryMonth) + currentDay + mod;

        var fullHashCode = todayDate.ToString("yyyyMMdd") + computedHash;

        return fullHashCode;
    }
}

I was writing unit test cases for this class and realized that AddDays() can throw an ArgumentOutOfRangeException, below line:

var expiryDate = todayDate.AddDays(expireInDays);

So I should write a test and i did write below test case:

    [TestMethod]
    public void GenerateHashCodeForExportLink_IncorrectDate_Throw()
    {
        try
        {
            HashCodeHelper.GenerateHashCodeForExportLink(new DateTime(2015, 1, 31), 501073, "001-345-673042");
            Assert.Fail("Exception is not thrown");
        }
        catch (ArgumentOutOfRangeException)
        {
        }
        catch (Exception)
        {
            Assert.Fail("Incorrect exception thrown");
        }
    }

The problem is, i do not know what to pass to cause AddDays() method to throw exception? I tried passing random dates e.g. 1-1-1800, 30-Jan-2015, etc.

I looked at AddDays() method implementation but could not make out. Any idea?