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https://issues.apache.org/jira/browse/ARROW-14442?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17482330#comment-17482330
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Dragoș Moldovan-Grünfeld commented on ARROW-14442:
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My understanding of this issue has evolved a bit.
bq. Set the timezone to local time without changing the integer value fo the
timestamp. We store whatever integer R passes to us (21600), with CST as the
timezone set. Display is then "1970-01-01 00:00:00 CST"
bq. This is surprising because we are asserting the local timezone when that is
not specified in R.
I think this is a 2 part problem:
# If the timezone information is missing in an R POSIXct vector, assume it is
the system timezone and pass this info to arrow without modifying the absolute
value (seconds since epoch). I think a _warning_ a maybe a bit too strong as a
condition when this happens so maybe a _message_ might be more suitable.
# Adjust the print method so that the displayed time matches the timezone
recorded as metadata.
The first part will be addressed by this Jira, while part 2 will be addressed
by https://issues.apache.org/jira/browse/ARROW-14567
> [R] Should we warn when converting timestamps with "" as tzone?
> ---------------------------------------------------------------
>
> Key: ARROW-14442
> URL: https://issues.apache.org/jira/browse/ARROW-14442
> Project: Apache Arrow
> Issue Type: Improvement
> Components: R
> Reporter: Jonathan Keane
> Assignee: Dragoș Moldovan-Grünfeld
> Priority: Major
> Labels: pull-request-available
> Time Spent: 3h 10m
> Remaining Estimate: 0h
>
> Form the comments, we've decided to go with option 3:
> * Set the timezone to local time without changing the integer value fo the
> timestamp. We store whatever integer R passes to us (21600), with CST as the
> timezone set. Display is then "1970-01-01 00:00:00 CST"
> This is surprising because we are asserting the local timezone when that is
> not specified in R.
> ============================================
> {{POSIXct}} in R can have timezones specified as {{""}} which is typically
> interpreted as the session local timezone.
> This can lead to surprising results like:
> {code:r}
> > Sys.timezone()
> [1] "America/Chicago"
> > as.integer(as.POSIXct("1970-01-01"))
> [1] 21600
> > Sys.setenv(TZ = "UTC")
> > as.integer(as.POSIXct("1970-01-01"))
> [1] 0
> > Sys.setenv(TZ = "Australia/Brisbane")
> > as.integer(as.POSIXct("1970-01-01"))
> [1] -36000
> {code}
> See also:
> https://stackoverflow.com/questions/69670142/how-can-i-store-timezone-agnostic-dates-for-sharing-between-r-and-python-using-p/69678923#69678923
>
> This runs counter to what timestamps without timezones are interpreted as in
> Arrow:
> https://github.com/apache/arrow/blob/03669438bbce53078616c7f943a63fb0c11db196/format/Schema.fbs#L333-L336
> > However, it may also be encoded into a Timestamp column with an empty
> > timezone. The timestamp values should be computed "as if" the timezone of
> > the date-time values was UTC; for example, the naive date-time "January 1st
> > 1970, 00h00" would be encoded as timestamp value 0.
> Critically in R, when {{as.POSIXct("1970-01-01 00:00:00")}} is run, the
> timestamp value is computed "as if" the timezone of the date-time values was
> the local timezone (and *not* UTC like the Arrow spec says).
> This can lead to some surprising results when converting these timezoneless
> timestamps from R to Arrow. Using {{as.POSIXct("1970-01-01 00:00:00")}} as an
> example, and presume US Central time. We have a few options:
> * Warn when the timezone is "" or not set that the behavior might be
> surprising
> We store whatever integer R passes to us (21600), with no timezone set.
> When someone sees this formatted, the times/dates will be what the time was
> at UTC ("1970-01-01 06:00:00")
> * Set the timezone to UTC without changing the integer value of the
> timestamp. We store whatever integer R passes to us (21600), with UTC as
> the timezone set. When someone sees this formatted, the times/dates will be
> in UTC ("1970-01-01 06:00:00 UTC") This might be surprising /
> counterintuitive because the timestamps will suddenly be different and will
> be based in UTC and not local time like people are expecting.
> * Set the timezone to local time without changing the integer value fo the
> timestamp. We store whatever integer R passes to us (21600), with CST as the
> timezone set. Display is then "1970-01-01 00:00:00 CST"
> This is surprising because we are asserting the local timezone when that is
> not specified in R.
> If someone is using a timestamp without tzone in R to represent a
> timezoneless timestamp, options 2 and 3 above violate that when it is put
> into Arrow. Whereas, if someone is using a timestamp that just so happens to
> be without a tzone but they assume it's in local time, option 1 leads to
> (very) surprising results
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