How to determine the startdate (monday) of a week with a predefined date?

My example date is 08.10.2010 (friday), so the startdate should be 04.10.2010 (monday).

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NSDate *today = [[NSDate alloc] init];
NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];

NSDateComponents *weekdayComponents = [gregorian components:NSWeekdayCalendarUnit fromDate:today];

NSDateComponents *componentsToSubtract = [[NSDateComponents alloc] init];
[componentsToSubtract setDay: 0 - ([weekdayComponents weekday] - 2)];

NSDate *beginningOfWeek = [gregorian dateByAddingComponents:componentsToSubtract toDate:today options:0];

NSLog(@"%@", beginningOfWeek);

The problem with this code-snippet is, that the week starts with sunday. This means, if I use the date 10.10.2010 (sunday) I also get 10.10.2010 as the start of the week, but it should be 04.10.2010 (monday). What is wrong with this code? Or are there other better solutions to solve this problem?