Convert.ToDateTime: how to set format

You should probably use either DateTime.ParseExact or DateTime.TryParseExact instead. They allow you to specify specific formats. I personally prefer the Try-versions since I think they produce nicer code for the error cases.

DateTime doesn't have a format. the format only applies when you're turning a DateTime into a string, which happens implicitly you show the value on a form, web page, etc.

Look at where you're displaying the DateTime and set the format there (or amend your question if you need additional guidance).

If value is a string in that format and you'd like to convert it into a DateTime object, you can use DateTime.ParseExact static method:

DateTime.ParseExact(value, format, CultureInfo.CurrentCulture);

Example:

string value = "12/12";
var myDate = DateTime.ParseExact(value, "MM/yy", System.Globalization.CultureInfo.InvariantCulture, System.Globalization.DateTimeStyles.None);

Console.WriteLine(myDate.ToShortDateString());

Result:

2012-12-01