Add make_date() feature for converting date-times into dates

[huftis]

I propose adding a convenience function make_date() for converting date-times to dates. One might by tempted to use as.Date(), and this works in many cases, but with the default options it gives surprising results in a few cases. Example:

x1 = as.POSIXct("2010-08-03 00:59:59.23")
x2 = as.POSIXct("2010-08-03 00:59:59.23", tz="Europe/London")
x3 = as.POSIXct("2010-11-03 00:59:59.23")
x4 = as.POSIXct("2010-11-03 00:59:59.23", tz="Europe/London")

The naïve user might expect as.Date() to return the 3rd of the month (August/November) for all of x1–x4. However, on my system (a Europe/Oslo timezone), I get:

> as.Date(x1)
[1] "2010-08-02"
> as.Date(x2)
[1] "2010-08-02"
> as.Date(x3)
[1] "2010-11-02"
> as.Date(x4)
[1] "2010-11-03"

It’s even more surprising if you have already used floor_date(x, "day") on the objects to remove the ‘time’ part.

The way to get the correct date (Date) corresponding to a date-time is either as.Date(format(x, "%Y-%m-%d")) or (preferably) as.Date(x, tz=attr(x, "tzone")):

> as.Date(x1, tz=attr(x1, "tzone"))
[1] "2010-08-03"
> as.Date(x2, tz=attr(x2, "tzone"))
[1] "2010-08-03"
> as.Date(x3, tz=attr(x3, "tzone"))
[1] "2010-11-03"
> as.Date(x4, tz=attr(x4, "tzone"))
[1] "2010-11-03"

So I propose adding such a convenience function make_date() to lubridate, that would run

as.Date(x, tz=attr(x, "tzone"))

for POSIXct and POSIXlt objects (and just return x for Date() objects).

I just based the name on the make_difftime() function; other names may better.

[hadley]

This seems more like a bug in base R, in my opinion.

[vspinu]

bug in base R,

I also think this should be addressed in base R. No need to duplicate existent functions. Would you bother firing a bug report?

I think make_date and make_datetime could serve as better names for existent ISOdate and ISOdatetime functions.

[huftis]

I’m pretty sure this will not be seen as bug by R Core. It’s documented and intentional. While I (and we) see it as a bug/misfeature, I see no chance of the default behaviour being changed in R base.

So the only chance of having a widely-used sensible and consistent datetime-to-date function would be having such a function in the lubridate package. (Of course, this is not a required function, as it’s easy to work around, but it would be very helpful having it as a convenience function. I like convenience functions. ☺)

[hadley]

@huftis oh good point, I didn't see that it was documented

@vspinu I think it's better to keep creation and coercion functions separate, but we could have as_date() and make_date()

[imanuelcostigan]

@huftis This annoys me greatly. I will submit a pull request.