iphone - Strange NSDate and NSDateFormatter behaviour -


i trying date week of year. code , runtime value being calculated:

enter image description here

why year not correct? tried other formats well, y-w , gave me same results. issue fix if use yyyy-w wrong month/date since year calculated differently then. information yyyy:

enter image description here

now date 04-29 correct 1 week 18 in "yyyy" case (see here: http://www.epochconverter.com/date-and-time/weeknumbers-by-year.php ) year correct when "yyyy" used (date wrong then).

so whats that?

-------- update 1 ---------

after adding solution offered in 1 of answers:

enter image description here

-------- update 2 ----------

so date 2000 same value "no year defined" (previously 1970). see here: http://openradar.appspot.com/12358210

-------- update 3 ----------

system information (for try replicating this).

xcode: version 4.6.2 (4h1003) ios: 6.1 osx: 10.8.3

-------- update 4 ----------

ios 5.1 provides different results when using this:

enter image description here

as if ios 5.1 not support "w" in nsdateformatter.

edit 1: answer doesn't explain results op seeing. see comments discussion more info

edit 2: peculiar results on machine. on ios 6, week doesn't respect first day of week setting in system preferences @ all:

ios 6 returns:

2000-1 -> 1st jan, 2000 (saturday)

2001-1 -> 6th jan, 2001 (saturday)

even if [calendar setfirstweekday: 3] , [dateformatter setcalendar: calendar], doesn't affect results @ (unlike ios 5)! bug in ios 6 nscalendar api. recommend using nsdatecomponents or calculating date yourself. should easy given info in answer - can assume iso-8601 (minimum 4 days in first week , week starts monday), unless of course, can ask user preference.

original answer

here's what's happening:

in locale, minimum number of days in week 5 , week starts on saturday.

assuming true, first week 5th jan (sat) - 11th jan 2013. , 18th week be, guessed it: 4th may 2013.

why? because apple follows unicode technical standard #35 version tr35-25 (on ios 6+).

f.4 week of year  values calculated week of year field range 1 53 gregorian calendar    (they may have different ranges other calendars). week 1 year first week  contains @ least specified minimum number of days year. weeks between  week 1 of 1 year , week 1 of following year numbered sequentially 2 52  or 53 (if needed). example, january 1, 1998 thursday. if first day of  week monday , minimum days in week 4 (these values reflecting iso  8601 , many national standards), week 1 of 1998 starts on december 29, 1997, ,  ends on january 4, 1998. however, if first day of week sunday, week 1 of  1998 starts on january 4, 1998, , ends on january 10, 1998. first 3 days of 1998  part of week 53 of 1997.

also, yyyy-w right format.


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