Set the first of the month of a NSDate

I'm using a NSDate that is incremented/decremented thanks to NSDateCompoments:

dateComponents = [[NSDateComponents alloc] init];
    self.开发者_StackOverflowdate = [gregorian dateByAddingComponents:dateComponents toDate:[NSDate date] options:0];

...

[dateComponents setDay:1];
                    self.date = [gregorian dateByAddingComponents:dateComponents toDate:self.date options:0];
                    [dateComponents setDay:0];

Now, I want to set the day to set the NSDate to the first of the month (and the first month of the year too), but I dont know how to do

Thanks a lot!!!

EDIT:

 NSDateComponents *dateComponentsSetFirst = [gregorian components:( NSYearCalendarUnit |
                                                              NSMonthCalendarUnit | NSDayCalendarUnit ) fromDate:self.date];

    switch (self.segmentedControl.selectedSegmentIndex) {
        case 0:
            requestTy = @"hours";
            break;
        case 1:
            requestTy = @"days";
            NSLog (@"avant %@",self.date);
            [dateComponents setDay:1];
            self.date = [gregorian dateFromComponents:dateComponentsSetFirst];
            [dateComponents setDay:0];
            NSLog (@"après %@",self.date);
            break;
        case 2:
            requestTy = @"months";
            NSLog (@"before %@",self.date);
            [dateComponents setMonth:1];
            self.date = [gregorian dateFromComponents:dateComponentsSetFirst];
            [dateComponents setMonth:0];
            NSLog (@"after : %@",self.date);
            break;
        default:
            break;
    }

give me

2011-08-17 11:23:34.365 e-mars[20942:207] avant 2011-08-17 09:23:30 GMT
2011-08-17 11:23:34.366 e-mars[20942:207] après 2011-08-16 22:00:00 GMT

when I pass in the case 1 !

and

2011-08-17 11:26:05.747 e-mars[20942:207] before 2011-08-16 22:00:00 GMT
2011-08-17 11:26:05.747 e-mars[20942:207] after : 2011-08-16 22:00:00 GMT

when I pass in the case 2 !

You are not adding components to a date. You simply initialize the dateComponents from a given date, set the day and month (and year if you want) and then get the dateFromComponents. Apple Date and Time Programming Guide

NSDateComponents *dateComponents = [gregorian components:( NSYearCalendarUnit |
NSMonthCalendarUnit | NSDayCalendarUnit ) fromDate:[NSDate date]];
[dateComponents setDay:1];
[dateComponents setMonth:1];
self.date = [gregorian dateFromComponents:dateComponents];