I conducted a quick test with the Cocoa Touch to see how NSHebrewCalendar works. I'm particularly interested in the month numbers. I used a date picker to easily change dates, and I passed it in to a method which logs the hebrew date, the hebrew month number, the hebrew year, and if the year is leap year. That looks like this:
BOOL isHebrewLeapYear = [self.calendar isHebrewLeapYear:[calendar hebrewYearForDate:[self.calendar workingDate]]];
NSLog(@"Hebrew Date:%@, Month Number: %i, %i is Leap Year: %i", [self.calendar stringFromHebrewDate:[self.calendar workingDate]], [self.calendar hebrewMonthNumberFromDate:[self.calendar workingDate]], [calendar hebrewYearForDate:[self.calendar workingDate]], isHebrewLeapYear);
The self.calendar object is a custom class. The workingDate property is an NSDate instance. Here are the relevant method declarations.
// Check if a given year is a leap year
- (BOOL) isHebrewLeapYear:(NSInteger)year{
return ((7 * year + 1) % 19) < 7;
}
//Get the hebrew year for a given date
- (NSInteger) hebrewYearForDate:(NSDate *)date{
NSCalendar *hebrewCalendar = [[[NSCalendar alloc] initWithCalendarIdentifier:NSHebrewCalendar] autorelease];
return [[hebrewCalendar components:NSYearCalendarUnit fromDate:date] year];
}
- (NSInteger) hebrewMonthNumberFromDate:(NSDate *)date{
NSCalendar *hebrewCalendar = [[[NSCalendar alloc] initWithCalendarIdentifier:NSHebrewCalendar] autorelease];
return [[hebrewCalendar components:NSMonthCalendarUnit fromDate:date] month];
}
Apparently hebrew leap years are handled as follows:
Does it sound like my experiment is producing accurate results? Is this behavior documented anywhere?
Month numbering starts at 1, with that month being "Tishri".
Correct.
In a nonleap year, Adar is month number 7, not 6.
Technically incorrect. (More on this below)
In a leap year, Adar I is number 6 and Adar II is 7.
Correct.
"Nisan" is always 8 and so on until "Elul", which is always 13.
Correct.
So what's up with Adar in non-leap years? I ran this code to find out:
@autoreleasepool {
NSDate *today = [NSDate date];
NSCalendar *hebrew = [[NSCalendar alloc] initWithCalendarIdentifier:NSHebrewCalendar];
NSDateComponents *diff = [[NSDateComponents alloc] init];
NSDateFormatter *f = [[NSDateFormatter alloc] init];
[f setDateFormat:@"d MMMM y"];
[f setCalendar:hebrew];
for (NSInteger i = 0; i < 19; ++i) {
NSDateComponents *comp = [[NSDateComponents alloc] init];
[comp setYear:5772 + i];
[comp setDay:1];
NSLog(@"============= %d ============", [comp year]);
for (NSInteger i = 1; i <= 13; ++i) {
[comp setMonth:i];
NSDate *d = [hebrew dateFromComponents:comp];
NSLog(@"%d: %@ (%@)", i, [f stringFromDate:d], d);
}
}
}
In a leap year, this will log what you would expect:
============= 5790 ============
1: 1 Tishri 5790 (2029-09-10 07:00:00 +0000)
2: 1 Heshvan 5790 (2029-10-10 07:00:00 +0000)
3: 1 Kislev 5790 (2029-11-08 08:00:00 +0000)
4: 1 Tevet 5790 (2029-12-07 08:00:00 +0000)
5: 1 Shevat 5790 (2030-01-05 08:00:00 +0000)
6: 1 Adar I 5790 (2030-02-04 08:00:00 +0000)
7: 1 Adar II 5790 (2030-03-06 08:00:00 +0000)
8: 1 Nisan 5790 (2030-04-04 07:00:00 +0000)
9: 1 Iyar 5790 (2030-05-04 07:00:00 +0000)
10: 1 Sivan 5790 (2030-06-02 07:00:00 +0000)
11: 1 Tamuz 5790 (2030-07-02 07:00:00 +0000)
12: 1 Av 5790 (2030-07-31 07:00:00 +0000)
13: 1 Elul 5790 (2030-08-30 07:00:00 +0000)
For each incrementation of the "month" date component, we get a different date. But when we run this in a non-leap year, we get this:
============= 5789 ============
1: 1 Tishri 5789 (2028-09-21 07:00:00 +0000)
2: 1 Heshvan 5789 (2028-10-21 07:00:00 +0000)
3: 1 Kislev 5789 (2028-11-19 08:00:00 +0000)
4: 1 Tevet 5789 (2028-12-19 08:00:00 +0000)
5: 1 Shevat 5789 (2029-01-17 08:00:00 +0000)
6: 1 Adar 5789 (2029-02-16 08:00:00 +0000)
7: 1 Adar 5789 (2029-02-16 08:00:00 +0000)
8: 1 Nisan 5789 (2029-03-17 07:00:00 +0000)
9: 1 Iyar 5789 (2029-04-16 07:00:00 +0000)
10: 1 Sivan 5789 (2029-05-15 07:00:00 +0000)
11: 1 Tamuz 5789 (2029-06-14 07:00:00 +0000)
12: 1 Av 5789 (2029-07-13 07:00:00 +0000)
13: 1 Elul 5789 (2029-08-12 07:00:00 +0000)
Here we see that have a month of 6 and 7 will both evaluate to Adar. Thus, Adar is both the 6th and the 7th month in non-leap years.
Also, since we know that the year 5790 is a leap year, we can deduce a simpler implementation of the -isHebrewLeapYear: method:
- (BOOL) isHebrewLeapYear:(NSInteger)year{
return year % 19 == 14;
}