I dont understand you but are you looking for a specified format for your datetime?
string dateAndTimeFormat = "yyyy.MM.dd HH:mm:ss:fff"; // example of format
string dateAndTime = yourdatetimevalue;
DateTime toDateTime = DateTime.ParseExact(dateTime, dateTimeFormat, System.Globalization.CultureInfo.InvariantCulture)
How formats are used: http://msdn.microsoft.com/en-us/library/az4se3k1(v=vs.71).aspx
EDIT 1
The tryparse returns true or false. False if it fails. Maybee that can be usefull? Otherwise you can set the culture before the tryparse, if you are able to do so.
DateTime.TryParse(dateString, culture, styles, out dateResult)
Check the examples here :http://msdn.microsoft.com/en-us/library/9h21f14e.aspx
Under remarks:
"This method tries to ignore unrecognized data, if possible, and fills in missing month, day, and year information with the current date. If s contains only a date and no time, this method assumes the time is 12:00 midnight. If s includes a date component with a two-digit year, it is converted to a year in the current culture's current calendar based on the value of the Calendar.TwoDigitYearMax property. Any leading, inner, or trailing white space character in s is ignored. The date and time can be bracketed with a pair of leading and trailing NUMBER SIGN characters ('#', U+0023), and can be trailed with one or more NULL characters (U+0000)."
Hope some of that helps.