On Jan 4, 2013, at 4:44 PM, Nick Zitzmann <n...@chronosnet.com> wrote:
> Short question: Given a year, week-of-year, and weekday in that week-of-year,
> what is the most reliable way to get the NSDate for that day even when the
> week-of-year is week 1? This is calendrical grey area, because there may be
> two week 1s in a year.
Short answer: Use NSYearForWeekOfYearCalendarUnit.
> Long question:
>
> We're using the NSCalendar class to get an NSDate given a year, week-of-year,
> and weekday number. For weeks 2 through 52 of the Gregorian year, NSCalendar
> gives us the correct date.
>
> But this falls apart when the week-of-year number is week 1, because many
> years on the Gregorian calendar have two week 1s. In 2013 AD, there are two
> week 1s if the first weekday is on a Sunday - this week, and the last week of
> the year. So when I tell it to find the date corresponding to Sunday of week
> 1 of 2013, I want NSCalendar to return December 30, 2012. That is the first
> Sunday of the first week of 2013. However, it actually returns December 29,
> 2013. That date is the Sunday of the **other** week 1 of 2013, the one that
> is the first week of 2014.
>
> What do I have to set in NSDateComponents to give NSCalendar a little more
> context so I get the correct date number? I already tried setting the month
> to 1 (January) in the components, and I got the same incorrect results.
If you want to specify dates using weekday+week+year, you need to use
NSYearForWeekOfYearCalendarUnit. yearForWeekOfYear is similar to year, except
the yearForWeekOfYear boundaries are adjusted so that a yearForWeekOfYear never
starts in the middle of a week.
For example, Monday December 31 2012 is:
year 2012, month 12, day 31
or
yearForWeekOfYear 2013, weekOfYear 1, weekday 2
(assuming firstWeekday is Sunday)
If your weekday+week+year specifications are coming from a human who doesn't
know about yearForWeekOfYear, then you'll need to do a bit of wrangling to find
the edge so you can translate year 2012 into yearForWeekOfYear 2013 when
necessary. It may be useful to get January 1st 2013 via year+month+day and
convert it to yearForWeekOfYear+weekOfYear+weekday (using -dateFromComponents:
then -components:fromDate:). January 1st 2013's weekday value tells you how
many of year 2012's days fall into yearForWeekOfYear 2013.
--
Greg Parker gpar...@apple.com Runtime Wrangler
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