[ARROW-14442] [R] fix behaviour when converting timestamps with "" as tzone

• Type: Improvement
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 8.0.0
• Component/s: R
• Labels: pull-request-available
• External issue URL: https://github.com/apache/arrow/issues/30005
• Assignee: Dragoș Moldovan-Grünfeld
• Reporter: Jonathan Keane
• Created: 22/Oct/21 17:32
• Updated: 11/Jan/23 08:40
• Resolved: 11/Apr/22 17:15

Description

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:

> 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

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

Issue Links

• is blocked by: ARROW-13168 [C++] Timezone database configuration and access
• is depended upon by: ARROW-15762 [R] Revisit binding_format_datetime and remove manual casting
• is related to: ARROW-14567 [C++][Python][R] PrettyPrint ignores timezone
• links to: GitHub Pull Request #12240