How to convert any date format to yyyy-MM-dd

string DateString = "11/12/2009";
IFormatProvider culture = new CultureInfo("en-US", true); 
DateTime dateVal = DateTime.ParseExact(DateString, "yyyy-MM-dd", culture);

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DateTime.ToString() Patterns

String Format for DateTime [C#]

string sourceDateText = "31-08-2012";
DateTime sourceDate = DateTime.Parse(sourceDateText, "dd-MM-yyyy")
string formatted = sourceDate.ToString("yyyy-MM-dd");

Convert your string to DateTime and then use DateTime.ToString("yyyy-MM-dd");

DateTime temp = DateTime.ParseExact(sourceDate, "dd-MM-yyyy", CultureInfo.InvariantCulture);
string str = temp.ToString("yyyy-MM-dd");