The following code changes the day from the given "date" for example: if you want to add or subtract 5 days from the date put a value 5 or -5 respectively for "timeSliceDay".

-(NSString *)getTimeSliceDate:(NSString *)date format:(NSString *)format returnFormat:(NSString *)returnFormat timeSliceDay:(int)timeSliceDay{
    NSDate *dated = [self dateFromString:date format:format];

    dated = [dated dateByAddingTimeInterval:timeSliceDay*24*60*60];
    return [self stringFromDate:dated format:returnFormat];
}

-(NSDate*)dateFromString:(NSString *)dateString format:(NSString *)format {
    NSDateFormatter *formatter = [[NSDateFormatter alloc]init];
    [formatter setDateFormat:format];
    return [formatter dateFromString:dateString];
}
-(NSString*)stringFromDate:(NSDate *)date format:(NSString *)format {
    NSDateFormatter *formatter = [[NSDateFormatter alloc]init];
    [formatter setDateFormat:format];
    return [formatter stringFromDate:date];
}