R: $ operator invalid for atomic vectors Error with POSIX

i may be missing something really obvious here, could you help me?

when i send a date object to seLocalize() function, it returns me as expected for the code that follows (i sent 14-mar-18 and returned 13-mar-18)

seLocalize <- function(dataTeste) {
    data <- as.POSIXlt(dataTeste) 
    if(data$wday != 0) {
        data <- data-86400
    }
    print(data)
}

Although, for following the piece of code, the same test returns "Error: $ operator is invalid for atomic vectors"

seLocalize <- function(dataTeste) {
    data <- as.POSIXlt(dataTeste)
    while(data$wday != 0) {
        data <- data-86400
    }
    print(data)
}

Why?

I'm having troubles believing this is a proper problem description since that date was in a format that would not be recognized.

> seLocalize <- function(dataTeste) {
+     data <- as.POSIXlt(dataTeste)
+     while(data$wday != 0) {
+         data <- data-86400
+     }
+     print(data)
+ }
> seLocalize("13-mar-18")
Error in as.POSIXlt.character(dataTeste) : 
  character string is not in a standard unambiguous format

So I tried sending what I thought was a possible revised function (for the somewhat obscure coercion induced error ) a proper date value for as.POSIXlt:

 seLocalize2 <- function(dataTeste) {
     data <- as.POSIXlt(dataTeste)
     while(data$wday != 0) {
         data <- as.POSIXlt(data-86400)  # coerce back to POSIXlt
         }
     print(data)
     }
> seLocalize2("2018-03-25")
[1] "2018-03-25 PDT"
> seLocalize2("2018-03-29")
[1] "2018-03-25 PDT"