//example convert time to string
NSDateFormatter *inputDateFormatter2 = [[NSDateFormatter alloc]init];
    [inputDateFormatter2 setDateFormat:@"yyyy-MM-dd HH:mm:ss"];
    NSDateFormatter *inputDateFormatter3 = [[NSDateFormatter alloc]init];
    [inputDateFormatter3 setDateFormat:@"yyyy/MM/dd"];
    //"work_time6" = "2017-10-20 17:43:42";
    NSDate *date = [inputDateFormatter2 dateFromString:aKarte[@"update_date"]];
    NSString *dateString = @"";
    if (date) {
        dateString = [inputDateFormatter3 stringFromDate:date];
    }