NSDate counting is not right. How to add a week without doing the math?

Question

I am storing all dates in a NSMutableArray with the format @"yyyy-MM-dd". I have a date and I am adding a week to that event until I reach the end of the year.

So, if I have the date 2016-05-05 it will add a week to that date and store it in the array until it reaches 2016-12-31.

The code I have works fine, the ONLY problem I have is that when it reaches November, it misses one day. Here is an example:

Date starts at 2016-10-17 I add a week and I get 2016-10-24. and then store it in Array. I add another week (2016-10-31) and store it in array. I add another week and I get 2016-11-06 instead of 2016-11-07.

I can't see why this happens. Here is the code:

  if([anEvent[3] isEqualToString:@"Every Week"])
  {
    //date from events
    NSDate *from = [dateFormatter dateFromString:anEvent[1]];
    //NSDate *to = [dateFormatter dateFromString:anEvent[2]];
    NSDate *nextWeek = from;

    while ([_calendar yearOfDate:nextWeek] == [_calendar yearOfDate:[NSDate date]]){
      nextWeek = [nextWeek dateByAddingTimeInterval:7*24*60*60];
      [dateFormatter setDateStyle:NSDateFormatterMediumStyle];
      [dateFormatter setDateFormat:@"yyyy-MM-dd"];
      NSString *dates = [dateFormatter stringFromDate:nextWeek];
      [everyWeekDates addObject:dates];
    };

   [everyWeekDates removeLastObject];
  }

Any ideas why this happens?

NOTE:

I discover that this happens when I add the date to the array in format @"yyyy-MM-dd". When I store the event date as a NSDate in the array it works fine.