@mishaelfash
one option would be
parse 10-Nov-2020 into a datetime with datetime.parsexact method
myDate = DateTime.ParseExact("10-Nov-2020","dd-MMM-yyyy", CultureInfo.InvariantCulture)
then use:
strJulianDate = String.Format("{0}/{1}", myDate.ToString("yyyy"), myDate.DayOfYear.ToString("000"))
Also have a look here: