I understand that `as.character.POSIXt()` had an overhaul in R 4.3
(https://github.com/wch/r-source/commit/f6fd993f8a2f799a56dbecbd8238f155191fc31b),
and I have come across a new behaviour and I wonder if it is unintended?
When calling `as.character.POSIXt()` on a vector that contains elements where
the time component is midnight (00:00:00), it drops the time component of that
element in the resulting character vector. Previously the time component was
retained:
In R 4.2.3:
```
R.version$version.string
#> [1] "R version 4.2.3 (2023-03-15)"
(t <- as.POSIXct(c("1975-01-01 00:00:00", "1975-01-01 15:27:00")))
#> [1] "1975-01-01 00:00:00 PST" "1975-01-01 15:27:00 PST”
(tc <- as.character(t))
#> [1] "1975-01-01 00:00:00" "1975-01-01 15:27:00”
```
In R 4.3.1:
```
R.version$version.string
#> [1] "R version 4.3.1 (2023-06-16)"
(t <- as.POSIXct(c("1975-01-01 00:00:00", "1975-01-01 15:27:00")))
#> [1] "1975-01-01 00:00:00 PST" "1975-01-01 15:27:00 PST”
(tc <- as.character(t))
#> [1] "1975-01-01" "1975-01-01 15:27:00”
```
This has consequences when round-tripping from POSIXt -> character -> POSIXt,
since `as.POSIXct.character()` drops the time component from the entire vector
if any element does not have a time component:
In R 4.2.3:
```
R.version$version.string
#> [1] "R version 4.2.3 (2023-03-15)"
(t <- as.POSIXct(c("1975-01-01 00:00:00", "1975-01-01 15:27:00")))
#> [1] "1975-01-01 00:00:00 PST" "1975-01-01 15:27:00 PST”
(tc <- as.character(t))
#> [1] "1975-01-01 00:00:00" "1975-01-01 15:27:00”
as.POSIXct(tc)
#> [1] "1975-01-01 00:00:00 PST" "1975-01-01 15:27:00 PST”
```
In R 4.3.1:
```
R.version$version.string
#> [1] "R version 4.3.1 (2023-06-16)”
(t <- as.POSIXct(c("1975-01-01 00:00:00", "1975-01-01 15:27:00")))
#> [1] "1975-01-01 00:00:00 PST" "1975-01-01 15:27:00 PST”
(tc <- as.character(t))
#> [1] "1975-01-01" "1975-01-01 15:27:00”
as.POSIXct(tc)
#> [1] "1975-01-01 PST" "1975-01-01 PST”
```
`format.POSIXt()` retains its old behaviour in R 4.3:
```
R.version$version.string
#> [1] "R version 4.2.3 (2023-03-15)"
(t <- as.POSIXct(c("1975-01-01 00:00:00", "1975-01-01 15:27:00")))
#> [1] "1975-01-01 00:00:00 PST" "1975-01-01 15:27:00 PST”
(tf <- format(t))
#> [1] "1975-01-01 00:00:00" "1975-01-01 15:27:00”
as.POSIXct(tf)
#> [1] "1975-01-01 00:00:00 PST" "1975-01-01 15:27:00 PST”
```
```
R.version$version.string
#> [1] "R version 4.3.1 (2023-06-16)"
(t <- as.POSIXct(c("1975-01-01 00:00:00", "1975-01-01 15:27:00")))
#> [1] "1975-01-01 00:00:00 PST" "1975-01-01 15:27:00 PST”
(tf <- format(t))
#> [1] "1975-01-01 00:00:00" "1975-01-01 15:27:00”
as.POSIXct(tf)
#> [1] "1975-01-01 00:00:00 PST" "1975-01-01 15:27:00 PST”
```
And finally, the behaviour of `as.POSIXct.character()` has not changed (it
previously did, and still does, drop the time component from all elements when
any element has no time):
```R.version$version.string
#> [1] "R version 4.2.3 (2023-03-15)"
as.POSIXct(c("1975-01-01", "1975-01-01 15:27:00"))
#> [1] "1975-01-01 PST" "1975-01-01 PST”
```
```R.version$version.string
#> [1] "R version 4.3.1 (2023-06-16)"
as.POSIXct(c("1975-01-01", "1975-01-01 15:27:00"))
#> [1] "1975-01-01 PST" "1975-01-01 PST”
```
I don’t know if this is a bug/regression in `as.character.POSIXt()`, or
intended behaviour. If it is intended, I think it would benefit from some more
comprehensive documentation.
Thanks very much,
Andy Teucher
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