c# fecha a cadena aaaa-mm-dd
var date_string = date.ToString("yyyy-MM-dd");
Formato DateTime.Ahora a aaaa-mm-dd c#
var dateString1 = DateTime.Now.ToString("yyyyMMdd");
var dateString2 = DateTime.Now.ToString("yyyy-MM-dd");
Console.WriteLine("yyyyMMdd " + dateString1);
Console.WriteLine("yyyy-MM-dd "+ dateString2);