NSDate and DateByAddingComponents problem

Question

I'm dealing about this function on my application. I have to show some different graphics from different periods. The period is choose by swiping the current graph.

I have some trouble by using the DateComponents. When I'm testing the app, I show the Date edited by the dateByAddingComponents, and it looks like that in the Console:

Date : 2009-12-14 10:06:13 GMT
Date : 2009-11-13 10:06:13 GMT
Date : 2009-10-12 09:06:13 GMT
Date : 2009-09-11 09:06:13 GMT
Date : 2009-08-10 09:06:13 GMT
Date : 2009-07-09 09:06:13 GMT
Date : 2009-06-08 09:06:13 GMT
Date : 2009-05-07 09:06:13 GMT
Date : 2009-04-06 09:06:13 GMT
Date : 2009-03-05 10:06:13 GMT
Date : 2009-02-04 10:06:13 GMT
Date : 2009-01-03 10:06:13 GMT
Date : 2008-12-02 10:06:13 GMT
Date : 2008-11-01 10:06:13 GMT
Date : 2008-09-30 09:06:13 GMT
Date : 2008-08-29 09:06:13 GMT

as you can see, the hour's changing sometimes, but the biggest problem is that there miss a month (the 10th in the year 2008).

There is the code implemented for those actions :

if (currentPosition.x > gestureStartPoint.x) {
        switch (self.segmentedControl.selectedSegmentIndex) {
            case 0:
                [dateComponents setDay:-1];
                self.date = [gregorian dateByAddingComponents:dateComponents toDate:self.date options:0];
                NSLog(@"Date : %@", date);
                break;
            case 1:
                //NSLog(@"before : %@", date);
                [dateComponents setMonth:-1];
                self.date = [gregorian dateByAddingComponents:dateComponents toDate:self.date options:0];
                NSLog(@"Date : %@", date);
                break;
            case 2:
                //NSLog(@"before : %@", date);
                [dateComponents setYear:-1];
                self.date = [gregorian dateByAddingComponents:dateComponents toDate:self.date options:0];
                NSLog(@"Date : %@", date);
                break;
            default:
                break;
        }

    }
    else if(currentPosition.x <= gestureStartPoint.x){
        switch (self.segmentedControl.selectedSegmentIndex) {
            case 0:
                [dateComponents setDay:1];
                self.date = [gregorian dateByAddingComponents:dateComponents toDate:self.date options:0];
                NSLog(@"Date : %@", date);
                break;
            case 1:
                [dateComponents setMonth:1];
                self.date = [gregorian dateByAddingComponents:dateComponents toDate:self.date options:0];
                NSLog(@"Date : %@", date);
                break;
            case 2:
                [dateComponents setYear:1];
                self.date = [gregorian dateByAddingComponents:dateComponents toDate:self.date options:0];
                NSLog(@"Date : %@", date);
                break;
            default:
                break;
        }
    }

How can I solve it???

Answer 1

How can I solve it?

By being more careful. From the logs you show us it is clear that you are decrementing the date by 1 month and 1 day every time. So naturally, when you reach 2008-11-01, the next date is 2008-09-30.

The one-hour difference in the time is because of daylight savings time in your time zone.