Don't. Instead, convert it to a DateTime using either the users current DateTime settings or an absolute string. Then keep it as a DateTime until you actually need to format it for a display, and convert it then:
string date = "01/11/2011 13:24:17";
DateTime dt;
if (DateTime.TryParseExact(date, "MM/dd/yyyy HH:mm:ss", CultureInfo.InvariantCulture,DateTimeStyles.None , out dt))
{
...
}
string output = dt.ToString("dd/MM/yyyy HH:mm:ss");