double origin = new DateTime(2003,1,1).ToOADate();
// the date you are counting from
// this will be expressed in DAYS.
double now = DateTime.Now.ToOADate();
// today's date -- or whatever your comparison date is.
// also expressed in days.
double diff = Math.Abs(now - origin);
// difference in days between both dates
double diffInSeconds = diff * 86400.0;
// conversion from days to SECONDS (24*60*60)