I was playing around with the Datetime.ParseExact method, and it wants an IFormatProvider...
It works inputting null, but what exactly does it do?
In adition to Ian Boyd's answer:
Also CultureInfo
implements this interface and can be used in your case. So you could parse a French date string for example; you could use
var ci = new CultureInfo("fr-FR");
DateTime dt = DateTime.ParseExact(yourDateInputString, yourFormatString, ci);