NSDictionary doesn't save itself to a .plist file

I've read a lot of q&a but they didn't solve my problem.

I wrote this method to save some data from my nib to a dictionary and then to a .plist

-(void)save{

NSString *saveFilePath = [self saveFilePath];

if ([[NSFileManager defaultManager]fileExistsAtPath:saveFilePath]) {

        NSMutableDictionary *dictionary = [[NSMutableDictionary alloc] initWithContentsOfFile:saveFilePath];
        //I Use the "today date" for key
        NSDate *today = [[NSDate alloc]init];
        NSMutableArray *array = [[NSMutableArray alloc]init];
        NSString *delta = [[NSString alloc]initWithFormat:@"%d",[kmNew.text intValue] - [kmOld.text intValue]];

        [array addObject:delta];
        [array addObject:[consumoKg text]];
        [array addObject:[consumoEuro text]];

        [dictionary setObject:array forKey:today];

    BOOL success = [dictionary writeToFile:[self saveFilePath] atomically:YES];
    NSLog(@"%d",success);
        [today release];
        [delta release];
        [array release];
        [dictionary release];
}
else{

    NSMutableDictionary *dictionary = [[NSMutableDictionary alloc] init];

    NSDate *today = [[NSDate alloc]init];
    NSMutableArray *array = [[NSMutableArray a开发者_如何转开发lloc]init];

    NSString *delta = [[NSString alloc]initWithFormat:@"%d",[kmNew.text intValue] - [kmOld.text intValue]];

    [array addObject:delta];
    [array addObject:[consumoKg text]];
    [array addObject:[consumoEuro text]];

    [dictionary setObject:array forKey:today];

    // Check if the value are store correctly into the dictionary

    for (NSDate *key in dictionary) {
        for (NSString *string in [dictionary objectForKey:key]) {
            NSLog(@"%@",string);
        }
    }

   BOOL success =  [dictionary writeToFile:[self saveFilePath] atomically:YES];
   NSLog(@"%d",success);

    [today release];
    [delta release];
    [array release];
    [dictionary release];
}
}

The saveFilePath method is the following:

- (NSString *)saveFilePath {
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *documentsDirectory = [paths objectAtIndex:0];
    return [documentsDirectory stringByAppendingPathComponent:kFilenameHistory];
}

The variables are stored correctly into the dictionary, but the return BOOL value "success" is 0.. Why?

I guess NSDictionary keys must be strings. NSDate is not allowed when saving to plist file.