NSCalendar dateFromComponents returns wrong date by 2

问题

I want to find out the date of the first week of a mouth in a year:

NSCalendar *calendar = [NSCalendar currentCalendar];
NSDateComponents *components = [[NSDateComponents alloc] init];
[components setYear:2013];
[components setMonth:1];
[components setWeekOfMonth:1];
[components setWeekday:1];
NSDate *newDate = [calendar dateFromComponents:components];
NSLog(@"%@",newDate);

What I get is:

2012-12-29 23:00:00 +0000

And when I compare to my mac calendar what I need to get is:

2012-12-31 23:00:00 +0000

Any suggestions?

回答1:

Problem might be setting the weekDay here is the working code

 NSCalendar *calendar = [NSCalendar currentCalendar];
    NSDateComponents *components = [[NSDateComponents alloc] init];
    [components setYear:2013];
    [components setMonth:1];
    [components setDay:1];

    NSDate *newDate = [calendar dateFromComponents:components];
    NSLog(@"%@",newDate); //2012-12-31 23:00:00 +0000

other alternate you could use NSGregorianCalendar, instead of currentCalender

NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
        NSDateComponents *comp = [gregorian components:(NSYearCalendarUnit | NSMonthCalendarUnit | NSDayCalendarUnit) fromDate:[NSDate date]];
[comp setYear:2013];
[comp setMonth:1];
[comp setDay:1];
NSDate *firstDayOfMonthDate = [gregorian dateFromComponents:comp];
NSLog(@"%@",firstDayOfMonthDate);  // 2012-12-31 23:00:00 +0000

回答2:

(I realize you've now worked out what was going on, but for the sake of future readers...)

Look at what you're doing here:

NSDateComponents *components = [[NSDateComponents alloc] init];
[components setYear:2013];
[components setMonth:1];
[components setWeekOfMonth:1];
[components setWeekday:1];

Let's take today (28th February 2013) as an example and see what we get after each step (hypothetically; I can't check this!):

• setYear:2013 - no change, as the year is already 2013
• setMonth:1 - change to January: 2013-01-28
• setWeekOfMonth:1 - change to the same day-of-week (Thursday) in the first week of January 2013: 2013-01-03
• setWeekday:1 - change to the Sunday in the same week: 2012-12-30

Now when you print out the local midnight of 2012-12-30, but in UTC, you're getting "2012-12-29 23:00:00 +0000" as presumably your local time zone is 1 hour ahead of UTC.

So as you've already established, you want setDay instead of setWeekOfMonth / setWeekday, assuming you really want "January 1st" rather than "the Sunday of week 1 of January".

来源：https://stackoverflow.com/questions/15136796/nscalendar-datefromcomponents-returns-wrong-date-by-2