DateTime dtTime1 = dateTimePicker1.Value.Date;
DateTime dtTime2 = dateTimePicker2.Value.Date;
int iTotalDays = 0;
int iTotalDaysInFirstYr = 0;
int iFirstMonth = dtTime1.DayOfYear;
if (DateTime.IsLeapYear(dtTime1.Year))
{
    iTotalDaysInFirstYr = 366 - iFirstMonth;
}
else
{
    iTotalDaysInFirstYr = 365 - iFirstMonth;
}               

for (int iCount = 0; iCount < (dtTime2.Year - dtTime1.Year - 1); ++iCount)
{
    if (DateTime.IsLeapYear(dtTime1.Year + iCount + 1))
    {
        iTotalDaysInFirstYr += 366;
    }
    else
    {
        iTotalDaysInFirstYr += 365;
    }
}

iTotalDays = iTotalDaysInFirstYr + dtTime2.DayOfYear;