为什么日期这样写：
> mydates <- c("2022-06-21","2022-06-024","2022-06-25","2022-06-27")
> mydates <- as.Date(mydates,format = "%Y-%m-%d")
> mydates
[1] "2022-06-21" "2022-06-02" "2022-06-25"
[4] "2022-06-27"
可以。

而这样写：
> mydates <- c("1974-03-27","1977-02-25","2021-06-05","2022-06-21")
> mydates <- as.Date(mydates,format = "%Y-%M-%D")
> mydates
[1] NA NA NA NA
返回的全是NA呢？