将序列号转换为R中的日期时的NA

Question

将Excel工作表导入R后，我发现所有日期格式都已更改为序列号。我尝试使用如下的非常简单的代码，但是我得到NA而不是日期。

可重复的例子是（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")

我在指定as.Date后测试了origin个别序列号并且工作正常。然而，当我尝试同时转换（df）的整个第一列时，它会给出那些NA的

我使用的是Mac OS 10.10和R 3.1.1