Any Day of the Week Using the Doomsday Rule

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Any Day of the Week Using the Doomsday Rule

An ingenious yet simple way to calculate the day of the week for any date was invented by John Conway called the Doomsday rule. I highly recommend reading the article by S.W. Graham which fully describes the algorithm. Basically by knowing which day of the week February 28 or 29 (leap year) falls on during a given century and which day of the week corresponds to that day for all the other months one can determine any day of the week. The following table summarizes which integers represent weekdays and what day represents the doomsday for the month and century.

Since the Gregorian Calendar repeats every 400 years one really only needs to remember four centuries. For example which day of week was November 5^th, 1955? (more importantly which movie uses that date?).

The COleDateTime class has date limits from Jan 1, 100 to Dec 31, 9999 and the CTime class has date limits from Jan 1, 1970 to Jan 18, 2038. The DoomsdayDate class can be used to find the day of the week during any range including B.C. dates.

DoomsdayDate

class DoomsdayDate
{

	//-----------------------------------------------------

	// JulianBeforeGregorian refers to the weird dates

	// between October 5-14, 1582 which were deleted from

	// the Gregorian calendar.  Technically the days never

	// existed.  The default is false which sets the date

	// as invalid.

	//-----------------------------------------------------

	DoomsdayDate(bool JulianBeforeGregorian = true);

	bool AD() { return ad_; }
	bool BC() { return !ad_; }

	// pass in ad = false for B.C. dates

	//--------------------------------------------------

	bool Set(int month, int day, int year, bool ad=true);

	// Find the first, second, third, or fourth

	// weekday in the specified month and year.

	// Only A.D. dates are supported.

	//----------------------------------------------

	bool SetFirst(int weekday, int month, int year);
	bool SetSecond(int weekday, int month, int year);
	bool SetThird(int weekday, int month, int year);
	bool SetFourth(int weekday, int month, int year);

	const char* WeekdayStr(int weekDay);

	// returns day of month

	//---------------------------

	int GetDay() { return day_; }

	// returns int month

	// 1 = January

	// 2 = February

	// ...

	//-------------------------------

	int GetMonth() { return month_; }

	// returns int year

	//-----------------------------

	int GetYear() { return year_; }

	// 0 == Sunday

	// 1 == Monday

	// 2 == Tuesday

	// 3 == Wednesday

	// 4 == Thursday

	// 5 == Friday

	// 6 == Saturday

	//------------------

	int Weekday();

	void Print();
}

DoomsdayDate Weekday Function

The core of the doomsday algorithm is the Weekday function.

int Weekday()
{
	int r = -1;
	int x = 0, y = 0;
	int ddcentury = -1;
	int ddmonth = DoomsdayMonth(month_);
	if( gregorian_ ) // Gregorian Calendar

	{
		int century = year_ - (year_ % 100);
		ddcentury = DoomsdayCentury(century);
		if( ddcentury < 0 ) return -1;
		if( ddmonth < 0 ) return -1;
		if( ddmonth > day_ )
		{
            weekday_ = (7 - ((ddmonth-day_) % 7 ) + ddmonth);
		}
        else
        {
            weekday_ = day_;
        }
		x = (weekday_ - ddmonth);
		x %= 7;
		y = ddcentury + (year_-century) + (floor((year_-century)/4));
		y %= 7;
		r = (x+y)%7;
	}
	else if( !ad_ ) // B.C -> AD Julian

	{
		int dd = -1;
		if( year_ > 699 )
		{
			dd = (year_ - (year_ % 700) + 701) - year_;
		}
		else
		{
			dd = (year_ - (year_ % 28) + 29) - year_;
		}
		if( dd > 0 )
		{
			ddcentury = (((dd - (dd % 100)) / 100) * 6)%7;
			x = ((dd%100)%7) + (int)floor((dd%100)/4)%7;
			if( ddmonth > day_ )
				y = ddmonth + day_;
			else
				y = day_ - ddmonth;
			y %= 7;
			x = ddcentury + x;
			x %= 7;
			r = (x+y)%7;
		}
	}
	else // Julian Calendar

	{
		ddcentury = (((year_ - (year_ % 100)) / 100) * 6)%7;
		x = ((year_%100)%7) + (int)floor((year_%100)/4)%7;
		if( ddmonth > day_ )
			y = ddmonth + day_;
		else
			y = day_ - ddmonth;
		y %= 7;
		x = ddcentury + x;
		x %= 7;
		r = (x+y)%7;
	}
	weekday_ = r;
	return weekday_;
}

DoomsdayDate Usage

The default constructor takes a boolean which indicates if the Julian calendar should be used prior to October 15^th, 1582 which defaults to true. Since October 5-14, 1582 were omitted from the Gregorian calendar they are not valid dates. If false is passed in all dates are synchronized with the Gregorian calendar.

The sample project takes the following arguments:

MM DD YYYY [BC]

where BC is optional to indicate if the date is B.C. It will also output the day of the week that COleDateTime will calculate for comparison purposes. Some interesting results of the sample program are below:

Legal U.S. Holidays

The following code uses the DoomsdayDate class to calculate which date all legal U.S. holidays occur on. It is contained in the LegalHoliday.h file.

namespace // *see note below

{
DoomsdayDate NewYears(int year)
{
    return DoomsdayDate(Month::JAN, 1, year);
}

DoomsdayDate MartinLutherKingJr(int year)
{
    DoomsdayDate dd;
    dd.SetThird(Weekday::MONDAY, Month::JAN, year);
    return dd;
}

DoomsdayDate WashingtonsBirthday(int year)
{
    DoomsdayDate dd;
    dd.SetThird(Weekday::MONDAY, Month::FEB, year);
    return dd;
}

DoomsdayDate MemorialDay(int year)
{
    DoomsdayDate dd;
    dd.SetFourth(Weekday::MONDAY, Month::MAY, year);
    return dd;
}

DoomsdayDate IndependenceDay(int year)
{
    return DoomsdayDate(Month::JUL, 4, year);
}

DoomsdayDate LaborDay(int year)
{
    DoomsdayDate dd;
    dd.SetFirst(Weekday::MONDAY, Month::SEP, year);
    return dd;
}

DoomsdayDate ColumbusDay(int year)
{
    DoomsdayDate dd;
    dd.SetSecond(Weekday::MONDAY, Month::OCT, year);
    return dd;
}

DoomsdayDate VeteransDay(int year)
{
    return DoomsdayDate(Month::NOV, 11, year);
}

DoomsdayDate ThanksgivingDay(int year)
{
    DoomsdayDate dd;
    dd.SetFourth(Weekday::THURSDAY, Month::NOV, year);
    return dd;
}

DoomsdayDate ChristmasDay(int year)
{
    return DoomsdayDate(Month::DEC, 25, year);
}
};

For this year the following outputs were generated:

Holidays 2002

=======================================
New Years is on a Tuesday 1/1/2002
Martin Luther King Jr. is on Monday 1/21/2002
Washington's Birthday is on Monday 2/18/2002
Memorial Day is on Monday 5/27/2002
Independence Day is on a Thursday 7/4/2002
Labor Day is on Monday 9/2/2002
Columbus Day is on Monday 10/14/2002
Veterans Day is on a Monday 11/11/2002
Thanksgiving Day is on a Thursday 11/28/2002
Christmas Day is on a Wednesday 12/25/2002

*Note on Unnamed Namespaces

The C++ standard states the following concerning anonymous or unnamed namespaces and their purpose.

Section 7.3.1.1 paragraph 2:

"The use of the static keyword is deprecated when declaring objects in a namespace scope; the unnamed-namespace provides a superior alternative. Although entities in an unnamed namespace might have external linkage, they are effectively qualified by a name unique to their translation unit and therefore can never be seen from any other translation unit."