How to get date from day of year

int a = 53;
var dt = new DateTime(DateTime.Now.Year, 1, 1).AddDays(a - 1);
string b = dt.ToString();

int dayOfYear = 53;
int year = DateTime.Now.Year; //Or any year you want
DateTime theDate = new DateTime(year, 1, 1).AddDays(dayOfYear - 1);
string b = theDate.ToString("d.M.yyyy");   // The date in requested format

Assuming you want the current year?

int a = 53;
DateTime date = new DateTime(DateTime.Now.Year, 1,1).AddDays(a -1);