"created_at" = "2012-09-21T12:41:26-0500"

NSArray *originalArray = @[
    @{@"created_at" : [NSDate dateFromISO8601String:@"2012-09-21T12:41:26-0500"]},
    @{@"created_at" : [NSDate dateFromISO8601String:@"2012-07-21T12:41:26-0500"]},
    @{@"created_at" : [NSDate dateFromISO8601String:@"2012-06-21T12:41:26-0500"]},
    @{@"created_at" : [NSDate dateFromISO8601String:@"2012-10-21T12:41:26-0500"]}
    ];

NSSortDescriptor *sortDescriptor =
    [[NSSortDescriptor alloc] initWithKey:@"created_at" ascending:NO];
NSArray *sortedArray =
    [originalArray sortedArrayUsingDescriptors:@[sortDescriptor]];