> 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
