• To convert Gregorian date to Julian format:
  Add corresponding displacement to day number.
  
  E.g.:    Convert Gregorian date 09/05/91 to Julian format
  
  
  • Displacement for month 09 for year 1991 (regular) is 243
  • Resultant Julian date is 91.248


• To convert Julian date to Gregorian format:
  
  Find nearest smaller displacement to Julian day;  line number gives month, day is a difference between Julian day and displacement.
  
  E.g.:    Convert Julian date 91.248 to Gregorian format
  
  
  • Nearest smaller displacement to 248 is 243 for year 1991 (regular) which gives month number 09
  • Difference between Julian day and displacement is 5 (248–243) which gives day number 05
  • Resultant Gregorian date is 09/05/91

Nostalgic note:  This simple table is inherited from the "good old days" of the mid 1970-th when Time Sharing was not just an "Option", but a real thing, and you had to reboot mainframe in the beginning of your own session in order to specify your private cataloged procedure library.  It was long before IBM "graciously" recognized this necessity (better late than never) and had introduced JCLLIB ORDER=(...) operator.  Rebooting at these ancient days involved entering current date, and, since the term "user friendly" was not invented yet, you had to enter date in the Julian format.  This was pain in the neck until somebody had come up with this conversion table.  Certain things never change — more than a quarter of a century later this table is still useful in dealing with some legacy applications.