while (days > 365)
    {
        if(IsLeapYear(year))
        {
            if (days > 366)
            {
                days -= 366;
                year += 1;
            }
        }
        else
        {
            days -= 365;
            year += 1;
        }
    }

For those without a programming background, the problem is that there is no “if” for day 366, just for days greater than 366 (which would be January 1 of 2009).  This means that the code gets caught in in infinite loop on the 366th day (December 31) of the year.  On January 1 of 2009, however, the “days” variable is set to 367, which causes the day to be set back to 1 (January 1) and the year to be incremented to 2009.  So the bug only lasts a day.

Programming 101 – be sure to test all of your cases, particularly those inside of loops.