Problem Statement

A company decides that whenever Xmas falls on a Sunday they will give their workers all extra paid holidays so that, together with any public holidays, workers will not have to work the following week (between the 25th of December and the first of January).

In what years between 2008 and 2121 will the 25th of December be a Sunday? Using any standard date handling libraries of your programming language; compare the dates calculated with the output of other languages to discover any anomalies in the handling of dates which may be due to, for example, overflow in types used to represent dates/times similar to   y2k   type problems.

Let's start with the solution:

PROGRAM YULETIDE
 
IMPLICIT NONE
  
INTEGER :: day, year
 
WRITE(*, "(A)", ADVANCE="NO") "25th of December is a Sunday in"
DO year = 2008, 2121
   day = Day_of_week(25, 12, year)
   IF (day == 1) WRITE(*, "(I5)", ADVANCE="NO") year
END DO
  
CONTAINS
 
FUNCTION Day_of_week(d, m, y)
   INTEGER :: Day_of_week, j, k, mm, yy
   INTEGER, INTENT(IN) :: d, m, y
  
   mm=m
   yy=y
   IF(mm.le.2) THEN
      mm=mm+12
      yy=yy-1
   END IF
   j = yy / 100
   k = MOD(yy, 100)
   Day_of_week = MOD(d + ((mm+1)*26)/10 + k + k/4 + j/4 + 5*j, 7)
END FUNCTION Day_of_week
  
END PROGRAM YULETIDE