Julian to Gregorian change

[nineball]

To mentally calculate the day of the week for Julian calendar requires changing the century code as follows:  deduct the first two numbers of the year from 18.  after deducting the highest multiple of 7 from this number the resulting number will be the correct century code to use for the Julian calendar.  i.e.   the code for 400 A.D. would be 18-4=14 deduct 14=0.   For 900 A.D.  18-9=9 deduct 7=2.  Other codes are the same as those used for Gregorian calendar.   Gregorian century codes which repeat continually every 400 yrs [ i.e. 6-4-2-0----6 if first two numbers are evenly divisible by 4  followed by 4-2-0-for each successive number] will work back to Sept 14 1752, then from Sept 2 1752 back to 4 A.D. the above adjustment is required.  [ie the century code for 1752 is 4 back to Sept 14 and changes to 1 on and  prior to Sept 2.