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?
