I'm working on moving some data from JD Edwards to a SQL Server database using SSIS and needed to deal with the way in which JDE stores dates.  The format is CYYDDD, where:

C = century, 1 for >= 2000 and 0 for < 2000
YY = the last two digits of the year
DDD = the number of the day.  Jan 1 = 1, Dec. 31 = 365 (or 366 in a leap year)

The .Net base class library has lots of good support for handling dates, but nothing as specific as the JD Edwards format, so I needed to write a bit of code to translate the JDE format to System.DateTime.  The function is below:

public static DateTime FromJdeDate(double jdeDate)
{
  DateTime convertedDate = DateTime.MinValue;

  if (jdeDate >= 30001 && jdeDate <= 200000)
  {
    short yearValue = (short)(jdeDate / 1000d + 1900d);
    short dayValue = (short)((jdeDate % 1000) - 1);
    convertedDate = DateTime.Parse("01/01/" + yearValue.ToString()).AddDays(dayValue);
  }
  else
  {
    throw new ArgumentException("The value provided does not represent a valid JDE date", "jdeDate");
  }

  return convertedDate;
}

 I'd love to take credit for this myself, but this is an adaptation of a TSQL UDF that I got from another consultant at the client site.