R `Inf` when it has class `Date` is printing `NA`

Question

Consider the following example:

structure(NA_real_, class = "Date")
## [1] NA
structure(Inf, class = "Date")
## [1] NA
is.na(structure(NA_real_, class = "Date"))
## [1] TRUE
is.na(structure(Inf, class = "Date"))
## [1] FALSE

Both are printing as NA. Is this the expected behavior or is this an error? It is very annoying to see NA for something that won't return TRUE for is.na().

Answer 1

This is expected behavior. What is printed is not what the object is. To be printed, the object needs to be converted to character. as.character.Date calls format.Date, which calls format.POSIXlt. The Value section of ?format.POSIXlt (or ?strptime) says:

The format methods and strftime return character vectors representing the time. NA times are returned as NA_character_.

So that's why NA is printed, because printing structure(NA_real_, class = "Date") returns NA_character_. For example:

R> is.na(format(structure(Inf, class = "Date")))
[1] TRUE
R> is.na(format(structure(NaN, class = "Date")))
[1] TRUE

If you somehow encounter these wonky dates in your code, I recommend you test for them using is.finite instead of is.na.

R> is.finite(structure(Inf, class = "Date"))
[1] FALSE
R> is.finite(structure(NaN, class = "Date"))
[1] FALSE