iOS Third Monday in January, for example

dejo said:

But since I'm going to need to do more dates than just MLK Day (Washington's Birthday, Memorial Day, etc.) with "trickier" relative dates, I think I may rework my method definition to look more like:

Code:

- (int)getDay:(NSString *)referenceString inMonth:(int)month forYear:(int)year

where referenceString could be something like "third" or "last" or, in the case of Fire Fighter's Memorial Day, "before or on the 9th".

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- (int)getReferenceString:(NSString *)referString dayOfWeek:(int)weekday inMonth:(int)month forYear:(int)year

int mlkDay = [self getReferenceString:@"third" dayOfWeek:2 inMonth:1 forYear:2014];

int mlkDay = [self getReferenceString:@"third" dayOfWeek:@"monday" inMonth:@"january" forYear:2014];

chown33 said:

I think the answer is the NSCalendar method dateFromComponents: and passing an NSDateComponents that sets the year, month, weekday, and weekdayOrdinal. The dateFromComponents: method returns an NSDate*, from which one can easily obtain the day-of-month.

From the discussion for weekdayOrdinal:

Weekday ordinal units represent the position of the weekday within the next larger calendar unit, such as the month. For example, 2 is the weekday ordinal unit for the second Friday of the month.
​

Setting the year and month are sufficient to identify a specific month. Setting weekday identifies which weekday one is interested in (e.g. the "Friday" in the "second Friday of the month" example). Setting weekdayOrdinal then identifies the ordinal number of the desired weekday.


I would do some experiments before committing to this.

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dejo said:

Experimenting now...

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chown33 said:

I vaguely recall that negative ordinals are interpreted relative to the end of the month. So -1 is the last Xday, -2 is the next-to-last Xday, etc. I could be wrong or misremembering.

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A solved problem:
http://en.wikipedia.org/wiki/Computus

I distinctly recall implementing Gauss's algorithm in Tiny Basic, on a Z80.

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ArtOfWarfare said:

Doesn't it follow a regular pattern on a different calendar? Will NSCalendar allow you to translate between them?

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- (NSDate *)getAnonymousGregorianEasterForYear:(int)year
{
	NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
	NSDateComponents *easterDateComponents = [[NSDateComponents alloc] init];
	[easterDateComponents setYear:year];
	
	int a = year % 19;
	int b = floor(year / 100);
	int c = year % 100;
	int d = floor(b / 4);
	int e = b % 4;
	int f = floor((b + 8) / 25);
	int g = floor((b - f + 1) / 3);
	int h = (19*a + b - d - g + 15) % 30;
	int i = floor(c / 4);
	int k = c % 4;
	int L = (32 + 2*e + 2*i - h - k) % 7;
	int m = floor((a + 11*h + 22*L) / 451);
	int month = floor((h + L - 7*m + 114) / 31);
	int day = ((h + L - 7*m + 114) % 31) + 1;
	
	[easterDateComponents setMonth:month];
	[easterDateComponents setDay:day];
	
	NSDate *easterDate = [gregorian dateFromComponents:easterDateComponents];
	
	return easterDate;
}

Duncan C said:

dejo,

I'm late to this party. I didn't discover this thread until just now. There is actually some very good info in Xcode about this sort of thing. Do a search on "Calendrical Calculations". That should lead you to a very informative article in the help system.

NSCalendar is an incredibly powerful class, especially when combined with NSDateComponents, NSDate, and NSDateFormatter.

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chown33 said:

The Chinese lunar calendar probably won't match the ecclesiastical lunar calendar used to calculate Easter. I certainly wouldn't expect it to. And there are differences between the Eastern Church (aka Eastern Orthodox) and the Western Church (Roman Catholic and Protestant), regarding the placement of March 21 (Julian vs. Gregorian calendars).

The link I posted on Computus contains links to the ecclesiastical new moon and full moon calculation.

FWIW, this is also why Easter Sunday isn't necessarily the Sunday after Passover, even though the Last Supper is a Passover meal. In short, see the Computus article; the calculation isn't as obvious as it may appear.

Things like this make conversion between decimal, binary, hex, IEEE-754, etc. seem like child's play, and remind me that there are long legacies of history, tradition, and accumulated error (e.g. Julian calendar) that have influenced us for centuries, and will likely continue to do so for many centuries or millenia more.

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moonman239 said:

I found the following code in a page on Stack Overflow.

Code:

- (NSDate *)getFirstDayOfTheWeekFromDate:(NSDate *)givenDate
{
    NSCalendar *calendar = [NSCalendar currentCalendar];


   NSDateComponents *components = [calendar components:NSYearCalendarUnit|NSMonthCalendarUnit|NSWeekCalendarUnit|NSWeekdayCalendarUnit fromDate:givenDate];
    [components setWeekday:2]; // 1 == Sunday, 7 == Saturday
    if([[calendar dateFromComponents:components] compare: curDate] == NSOrderedDescending) // if start is later in time than end
    {
        [components setWeek:[components week]-1];
    }

    return [calendar dateFromComponents:components];
}

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NSDate *today = [[NSDate alloc] init];
NSCalendar *gregorian = [[NSCalendar alloc]
          initWithCalendarIdentifier:NSGregorianCalendar];
 
// Get the weekday component of the current date
NSDateComponents *weekdayComponents = [gregorian components:NSWeekdayCalendarUnit
          fromDate:today];
 
/*
Create a date components to represent the number of days to subtract from the current date.
The weekday value for Sunday in the Gregorian calendar is 1, so subtract 1 from the number of days to subtract from the date in question.  (If today is Sunday, subtract 0 days.)
*/
NSDateComponents *componentsToSubtract = [[NSDateComponents alloc] init];
[componentsToSubtract setDay: 0 - ([weekdayComponents weekday] - 1)];
 
NSDate *beginningOfWeek = [gregorian dateByAddingComponents:componentsToSubtract
          toDate:today options:0];
 
/*
Optional step:
beginningOfWeek now has the same hour, minute, and second as the original date (today).
To normalize to midnight, extract the year, month, and day components and create a new date from those components.
*/
NSDateComponents *components =
     [gregorian components:(NSYearCalendarUnit | NSMonthCalendarUnit |
          NSDayCalendarUnit) fromDate: beginningOfWeek];
beginningOfWeek = [gregorian dateFromComponents:components];