Hi,

 x1 <- c(as.Date("20110101","%Y%m%d"),as.Date("2012-01-01","%Y-%m-%d")); 
 x2 <- c("1","2"); 

ddf <- data.frame(x=x1,y=x2); 
ddf[["y"]] <- as.factor(ddf[["y"]])

 write.arff(ddf, file="D:/ddf.arff") 

Content of ddf.arff is

@relation ddf
@attribute x numeric
@attribute y {'1','2'}
@data
2011-01-01,'1'
2012-01-01,'2'


Here x is of type Date, but write.arff write it as numeric, but the actual content is string, the result file is not with valid ARFF file format(Weka can't read it in).
I think write.arff should write it as @attribute x DATE "yyyy-MM-dd" 


Regards





Xiaobo Gu
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