string s = "20170517";
s = s.Insert(6, "-").Insert(4, "-");
DateTime d = Convert.ToDateTime(s);
d = d.AddDays(1);
s = d.ToString("yyyyMMdd");
DateTime.ParseExact("20170517", "yyyyMMdd", null)
string str = "20170809";
DateTime dt = DateTime.ParseExact(str.Trim(), "yyyyMMdd", null);
dt = dt.AddDays(1);
str = dt.ToString("yyyyMMdd");
string s="20170517";
DateTime dt = Convert.ToDateTime(s);
dt = dt.AddDays(1);
s = dt.ToString();