// Demonstrate Zellers Congruence
//    Calculates the day (name) for a given date

DIM weekdays$(7)
FOR i = 0 TO 6 CYCLE
  READ weekdays$(i)
REPEAT

PRINT "Weekday calculator"
INPUT "Day   ? ", day
INPUT "Month ? ", month
INPUT "Year  ? ", year
weekday = FN zeller(day, month, year)
PRINT day;  "/";  month;  "/";  year;  " is ";  weekdays$(weekday)
END


DEF FN zeller(day, month, year)
LOCAL t1, t2, t3, t4
IF month < 3 THEN
  year = year - 1
  month = month + 12
ENDIF
t1 = INT (((month + 1) * 26) / 10)
t2 = INT (year / 4)
t3 = INT (year / 100)
t4 = INT (year / 400)
 = (day + t1 + year + t2 + 6 * t3 + t4) MOD 7
DATA "Saturday", "Sunday", "Monday", "Tuesday"
DATA "Wednesday", "Thursday", "Friday"