I think the world is headed towards using canonical UTC timestamps (some period of time from the unix epoch at UTC) so that timestamps can be interpreted as absolute times without requiring additional metadata. This is not yet universal yet, but it seems to me to be where things are heading (and what new systems do)
This feels like the same basic transition the world did from "strings" with encodings as metadata which can be messed up to "UTF-8" Thus, I prefer Antoine's interpretation that timestamp values are always relative to UTC and the timezone metadata can be used to render them to local times if desired Andrew On Mon, Jun 14, 2021 at 3:25 PM Weston Pace <weston.p...@gmail.com> wrote: > > So it's wrong to put "timezone=UTC", because in Arrow, the 'timezone" > field > > means, "how the data is *displayed*." The data isn't displayed as UTC. > > I don't think users will generally be using Arrow to format timestamps > for display to the user. However, if it is, the correct thing to do > here would be to store and transport timestamps with timezone=UTC. > Then, when it comes time to display, first convert to timezone=local > and then convert to string. > > > If you parse a timestamp string, then you can extract all of the fields > > (including hour and day) from the resulting int64 values and they will be > > the same as they appeared in the strings. Many users never need to worry > > about time zone isn’t their analyses. > > If Arrow required a timezone (didn't allow naive timestamps) then > users that truly don't care about the timezone could simply specify > UTC or any other timezone and carry on with their analysis. > > Personally, I think the safest option would be to only allow > timestamps to be stored with a timezone. I'd agree with Antoine's > earlier point and say that the timezone should always be supplied at > the boundary. However, it may be too late for that. > > Given that we have "timestamps without timezone" it seems to me the > safest thing is to consider them as naive and fail any function that > required the time zone. So +1 for Joris' interpretation. Yes, this > renders them useless for any purpose other than pass-through. If a > user truly wants to do something with them then it seems the burden > should be on the user to supply a timezone and not for Arrow to infer > anything. > > -Weston > > > On Mon, Jun 14, 2021 at 9:12 AM Joris Van den Bossche > <jorisvandenboss...@gmail.com> wrote: > > > > On Mon, 14 Jun 2021 at 17:57, Antoine Pitrou <anto...@python.org> wrote: > > > > > > ... > > > > > > Joris' interpretation is that timestamp *values* are expressed in an > > > arbitrary "local time" that is unknown and unspecified. It is therefore > > > difficult to exactly interpret them, since the timezone information is > > > unavailable. > > > > > > (I'll let Joris express his thoughts more accurately, but the gist of > > > his opinion is that "can be thought of as UTC" is only an indication, > > > not a prescription) > > > > That's indeed correct. One clarification: you can interpret them as > > is, and for many applications this is fine. It's only when you want to > > interpret them as an absolute point in time that the user needs to > > supply a timezone to interpret them. > > > > For the rest, Wes' responses already cover my viewpoint (as a pandas > > maintainer, I of course have a similar perspective on this looking at > > this from the pandas implementation he wrote). > > > > An additional source that explains the "local semantics" of naive > > timestamps well IMO, and especially explains the "can be thought of as > > UTC without being UTC" aspect, is the parquet format docs: > > > https://github.com/apache/parquet-format/blob/master/LogicalTypes.md#local-semantics-timestamps-not-normalized-to-utc > > (it's of course about Parquet and not Arrow, but the explanation is > > relevant for the Arrow spec as well). > > > > Joris >
