convert C# date time to string and back

You should use the roundtrip format specifier “O” or “o” if you want to preserve the value of the DateTime.

The “O” or “o” standard format specifier represents a custom date and time format string using a pattern that preserves time zone information. For DateTime values, this format specifier is designed to preserve date and time values along with the DateTime.Kind property in text. The formatted string can be parsed back by using the DateTime.Parse(String, IFormatProvider, DateTimeStyles) or DateTime.ParseExact method if the styles parameter is set to DateTimeStyles.RoundtripKind.

Using your code (apart from changing the format string):

const string FMT = "O";
DateTime now1 = DateTime.Now;
string strDate = now1.ToString(FMT);
DateTime now2 = DateTime.ParseExact(strDate, FMT, CultureInfo.InvariantCulture);
Console.WriteLine(now1.ToBinary());
Console.WriteLine(now2.ToBinary());

I get:

-8588633127598789320
-8588633127598789320