Perpetual julian or gregorian calendar - has years classified in seven groups recorded in double-entry tables

摘要

The calendar allows the day of the week to be found ad infinitum for any date in the Julian or Gregorian calender, without calculation. The years are classified into 7 groups designated A - G, and are recorded in double-entry tables, which are seven in number for all the years of the Julian and four in number for all those of the Gregorian calendar. The groups of leap-years can be shown in red ink, and those of the remainder in black, blue or another colour. The perforated calendar panel can have two similar groups of windows, one above the other, and with symbols in different colours, one of them red.