I have the following function
DateTime fromDateParam = DateTime.ParseExact(Convert.ToString(DateTime.MinValue),\"dd.MM.yyyy HH:mm:ss\",null);
Well you're converting the original time to a string using the default formatting, but then you're specifying custom formatting for the parsing.
If you specify a format string using DateTime.ToString(format)
and keep the format consistent, it works fine:
string formatString = "dd.MM.yyyy HH:mm:ss";
string text = DateTime.MinValue.ToString(formatString);
Console.WriteLine(text);
DateTime fromDateParam = DateTime.ParseExact(text, formatString, null);
In other words (continuing Skeet's answer), Convert.ToString(DateTime.MinValue)
is based on current/default CultureInfo, etc.