Create dbf file from C # - DateTime field

Question

I used this library in my project. Library so very good, but i have some problems with format DateTime. I create method in static class DbfFieldDescriptors for Date format

public static DbfFieldDescriptor GetDateField(string fieldName, byte length = 8)
{
  return new DbfFieldDescriptor(fieldName, 'D', length, (byte) 0);
}

and i trying map field in my file:

var dt = DbfFieldDescriptors.GetDateField("dt", 8);
Func<ExportDBF, object> dtMap = exportDbf => exportDbf.dt.Date.ToString("yy-MM-dd");

and this is not working, i.e. this is not write to file, field is empty

Has anyone encountered such a problem? Can you help me with this?) I would be very grateful!

Thank you!

full code for writing to file (in method GetExportDbf() i'm retrieve data from db):

var list = GetExportDbf(dateStart, dateEnd, accId);
var ndoc = DbfFieldDescriptors.GetStringField("ndoc", 10);
var dt = DbfFieldDescriptors.GetDateField("dt", 8);

var columns = new List<DbfFieldDescriptor>() 
{
   ndoc, dt
}; 

Func<ExportDBF, object> ndocMap = exportDbf => exportDbf.ndoc;
Func<ExportDBF, object> dtMap = exportDbf => exportDbf.dt.Date.ToString("yy-MM-dd");

var mapping = new List<Func<ExportDBF, object>>()
{
   ndocMap, dtMap
};

var result = DbfFileFormat.GetByteArray(list, mapping, columns, Encoding.Default);
return result;