Add NA_Date_ and 0-length Date helpers

[AshesITR]

Sometimes I find the need to explicitly set a date to NA (or specify an empty Date vector).
The former would be

NA_Date_ <- structure(NA_real_, class = "Date")

This is especially useful in conjuction with dplyr::if_else wich will error out on
dplyr::if_else(FALSE, NA_real_, lubridate::today()) and any other NA_...

The latter would be

Date <- function(length = 0L)structure(rep(NA_real_, length), class = "Date")

wich is very useful in allocating an empty data.frame with a date column.

[vspinu]

This is a good idea but it's so fundamental that should be tried with base R first. I could add NA_date_ and Date to lubridate but then I will have to add NA_POSIXct_ and POSIXct for consistency. I feel that all these belong in R proper.

[vspinu]

I decided to add these. We already do is.Date and is.POSIXct. Even if accepted into the core it almost surely won't be treating the tz argument as we want it - "UTC" by default.

[yakyak462]

Hey -- sorry if this is obvious. But what is the workaround to not having NA_Date_ and needing to use if_else? The base R ifelse seems to breaks a lot of lubridate features.

[AshesITR]

@yakyak462 See the original issue: NA_Date_ <- structure(NA_real_, class = "Date").

[djbirke]

@vspinu Would it be possible to also add NA_Duration_ (and possibly the equivalent for other timespan classes)? (I am happy to open a new issue for this, if you prefer.)

[vspinu]

@djbirke yes that would be useful. PRs more than welkome.