将excel表格导入R后，发现所有日期格式都改成序号了。我尝试使用如下非常简单的代码，但是我得到的是 NA 而不是日期。
可重现的例子是（df）：
structure(list(X1 = c("31020", "31021", "31022", "31023", "31026",
"31027"), X2 = c("197.95000000000002", "186.8", "190.51", "195.16",
"199.81", "202.59"), X3 = c("90.070000000000007", "89.820000000000007",
"87.36", "87.61", "90.56", "89.820000000000007"), X4 = c("20.43",
"20.43", "20.43", "20.43", "20.43", "20.43"), X5 = c("24.54",
"23.97", "23.97", "24.38", "24.38", "24.38")), .Names = c("X1",
"X2", "X3", "X4", "X5"), row.names = c(NA, -6L), class = "data.frame")
我使用的代码是
df[,1]<-as.Date(df[,1], "%Y-%m-%d")
我as.Date在指定后用单独的序列号进行了测试origin，它工作正常。但是，当我尝试一次转换 (df) 的整个第一列时，它会给出那些 NA
我在 Mac OS 10.10 和 R 3.1.1