Use Date's tz() when converting with to_posixct()

[AdamSpannbauer]

Issue

Currently, to_posixct() assumes tz = "UTC" when converting a Date type. However, tz() checks for the "tzone" attribute. It is possible to run into dates with the "tzone" attribute and this can lead to unexpected results from to_posixct()

Example of unexpected results when using time_floor():

# Create a date with tzone attribute
x <- as.Date("2020-01-01")
attr(x, "tzone") <- "America/New_York"

packageVersion("timechange")
# [1] ‘0.1.1.9000’

# This version assumes date to be UTC in
# the `date_to_posixct()` conversion
timechange::time_floor(x, "months", 1)
# [1] "2019-12-31"

Proposed fix

Change to_posixct()'s call to date_to_posixct() to use tz(time) instead of assuming UTC. If the Date doesn't have a "tzone" attribute UTC will still be the default.

# after changing to_posixct() usage of date_to_posixct to:
# date_to_posixct(time, tz = tz(time))
timechange::time_floor(x, "months", 1)
# [1] "2020-01-01"

[vspinu]

Makes sense indeed. Thanks!

[vspinu]

Actually this simple solution is not quite enough. First, we don't actually want to end up with as.POSIXlt in as.Date for performance reasons. Second, the tzone argument is not propagated to the end result. Third, there are quite a few other places where this seems to be a problem (pretty much all the places where date is converted to posix). Let me have a closer look