NSDateFormatter issue involving week numbers

问题:

On my machine (regional settings United States), the default "short" date format is set to "1/5/13" (month/day/year).

In the System Preferences, I have appended to it a week number, "1/5/13 1".

My problem is this code, where I try to convert a string to a date:

NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setDateStyle:NSDateFormatterShortStyle];
NSDate *date = [dateFormatter dateFromString:@"1/5/13 1"];
NSLog(@"Date: %@", date);

On my machine, this prints:

Date: 2000-01-01 05:00:00 +0000

That's 2000, which is not even close to 2013.

What is causing this problem?

回答1:

I'm late to this discussion, and have seen a number of solutions offered, but what I see wrong with them all is that two incompatible date format elements are being used together. Your original approach failed because the string from which you're attempting to get your date doesn't match what NSDateFormatterShortStyle provides for.

If you're going to evaluate the week of the year, then you've got to use the capitalized form of the year; this provides for a "Week of Year" kind of calendar.

Let's re-work your original code a bit to include the new format:

NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setDateFormat:@"MM/dd/YY w"];
NSDate *date = [dateFormatter dateFromString:@"1/5/13 1"];
NSLog(@"Date: %@", date);

Note that I'm using that capitalized form for the year. With 1 for the week of the year, I get this:

Date: 2013-01-05 08:00:00 +0000

and with 2 for the week of the year:

Date: 2013-01-12 08:00:00 +0000

and with 3:

Date: 2013-01-19 08:00:00 +0000

Makes sense, doesn't it? The day in the date increments by seven days with each increment by one of the week of year. The time's dorked, of course, but we never discussed anything to evaluate in that area, did we?

回答2:

NSDateComponents *cmp=[[[NSDateComponents alloc]init]autorelease];
[cmp setWeek:1];
NSCalendar *cal=[[NSCalendar alloc]initWithCalendarIdentifier:NSGregorianCalendar];
NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
NSDate *dte=[NSDate date];
NSDate *weekDate=[cal dateByAddingComponents:cmp toDate:dte options:0];
[dateFormatter setDateFormat:@"MM/dd/YY"];
NSString *strDate=[dateFormatter stringFromDate:weekDate];
NSLog(@"Date: %@", strDate);

回答3:

You need to use dateFromString method of NSDateFormatter...

NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setDateFormat:@"mm/dd/yyyy"];
NSDate *myDate = [dateFormatter dateFromString:@"1/5/2013"];
NSLog(@"Date: %@", myDate);

If you want to play with week number then use NSCalendar and NSDateComponents based on your need... (refer this code snippet)

NSCalendar *cal = [NSCalendar currentCalendar];
NSDateComponents *components = [cal components:NSWeekCalendarUnit fromDate:date];
NSInteger week = [components week];

I hope this helps.