To save others the time:
c#
var scientificNotationText = textFieldFromEfuFileLine;
// FileTimes are based solely on nanoseconds.
long fileTime = 0;
long.TryParse(scientificNotationText, NumberStyles.Any, CultureInfo.InvariantCulture, out fileTime);
var dateModified = DateTime.FromFileTime(fileTime);
Enjoy