Question

I am writing a program to display the Day for the given date. (E.g. Thursday for 1/1/1970.)
While attempting that I have some problems.

This is my program.

/*Concept: 
 The noOfDays variable will count the no of days since 01/01/0001. And this will be Day one(1). E.g 
 noOfDays = 365 for 01/12/0001 and noOfDays = 731 for 01/01/0003 and so on....
 Since I taken Monday as 01/01/0001,the day of the specific date can be achieved by switching(noOfDays%7)
 E.g. 365 % 7 = 1. So, the 365th day is monday...
*/

import java.util.Scanner;
class DayDisplayer
{
    long noOfDays;
    int month;
    int days;
    int year;
    Scanner scan;
    int countYear;
    int countMonth;

    DayDisplayer()
    {
        scan = new Scanner(System.in);
        noOfDays = 0;
        System.out.println("Enter the date please");

        days = scan.nextInt();
        month = scan.nextInt();
        year = scan.nextInt();

        System.out.println("Your Date is:  "+days+"/"+month+"/"+year);



    }

    boolean leapYearCheck(int year)
    {
        if( ((year%100 == 0) && (year%400 != 0)) || (year%4 != 0) )  // when it is divisible by 100 and not by 400.  
            return false;
        else
            return true;

    }

    boolean isThere31Days()
    {
        if ( ( (countMonth >> 3) ^ (countMonth & 1) ) == 1 ) 
            return true;
        else 
            return false;

    }

    void getDaysThatInDays()    
    {
        noOfDays += days;
    }

    void getDaysThatInMonth()
    {

        countMonth = 1;

        while(countMonth<month)
        {
            if( countMonth == 2 )
                if( leapYearCheck(year) == true)
                    noOfDays += 29;
                else 
                    noOfDays += 28;
            else
               if ( isThere31Days()) 
                   noOfDays += 31;
               else 
                   noOfDays += 30;

            countMonth++;
        }                   



    }

    void getDaysThatInYear()
    {
        countYear = 1;

        while(countYear<year)
        {
            if( leapYearCheck(countYear)== true )  
                    noOfDays += 366; 
                else 
                    noOfDays += 365;

            countYear ++;
        }
    }

    void noOfDaysAndDayName()
    {
        System.out.println("The noOfDays is"+noOfDays);

        System.out.println("And Your Day is :"); 

        int day;

        day = (int)(noOfDays%7);
        switch(day)
        {

        case 0:
            System.out.println("Sunday");
            break;
        case 1:
            System.out.println("Monday");
            break;
        case 2:
            System.out.println("Tuesday");
            break; 
        case 3:
            System.out.println("Wednesday");
            break;
        case 4:
            System.out.println("Thursday");
            break;
        case 5:
            System.out.println("Friday");
            break;
        case 6:
            System.out.println("Saturday");
            break;
        }

    }

}// end of MonthToDate class

public class Main
{
    public static void main(String args[])
    {

        DayDisplayer  dd= new DayDisplayer();

        dd.getDaysThatInYear();
        dd.getDaysThatInMonth();
        dd.getDaysThatInDays();

        dd.noOfDaysAndDayName();


    }



}

In this code ,when I take 01/01/0001 as Monday and calculated the other days. And I am getting correct answers.

But In http://www.timeanddate.com website they took 01/01/0001 as Saturday.
But still for the other recent dates (say 17/7/2011) they are giving the correct answer (as Sunday).

I could guess these lags are due to the migration of Julier system to Gregorian system.

But I have a doubt whether my approach of calculating the Days from 0001 year is correct or not?

Also I have a doubt about whether to take 01/01/0001 date is Monday or Saturday.
(And If I took Saturday as my first day, I am getting wrong answers.)

someone please explain.

Thanks in advance.

Answer 1

One problem could be highlighted by an off-by-one error your opening comment:

The noOfDays variable will count the no of days since 01/01/0001. E.g 
noOfDays = 365 for 01/12/0001 and noOfDays = 731 for 01/01/0003 and so on.

That says that 0001-01-01 will be day 0 (0 days since the reference date), which in turn means that 0001-12-31 on your proleptic Gregorian calendar will be 364, not 365.

You can adjust this in either of two ways:

• Define that 0001-01-01 will be day 1 (whereupon the rest of the quoted fragment of comment is correct).
• Change the comment values to 364 and 730.

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Another problem with reverse engineering dates back that far is understanding the ‘proleptic’ Gregorian calendar. The term strictly means ‘the representation of a thing as existing before it actually does or did so’, and applies forward, but the term is also used in calendrical calculations to refer to applying the present rules backwards in time. The issue at hand is that the Gregorian calendar with its rules for leap years (divisible by 400, or divisible by 4 but not divisible by 100) did not exist in year 1. And the question is: did the web sites you referred to compensate for the switch between the Julian calendar and the Gregorian? (Underlying that are a lot of complications; the calendar was exceptionally volatile, even in the Roman Empire, between about 55 BCE and 200 CE.)

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An excellent book for considering matters of date is Calendrical Calculations.