[QGIS-Developer] QGIS, DateTime and TimeZones

[Nyall Dawson]

On Tue, 8 Sep 2020 at 23:40, Richard Duivenvoorde <rdmailings at duif.net> wrote:
>
> Hi Devs,
>
> I was hitting some timezone issues when I received some csv data in which there are timestamps like:
> "2020-07-25 20:21:38 UTC"
> Apparently if you read this into QGIS, the UTC IS interpreted, and you get a DateTime in your data which is (in my case) -2 hours(CET) off (well in some parts of QGIS)
>
> This plays havoc if you for example are going to use this data in the timecontroller or expressions, see
> https://github.com/qgis/QGIS/issues/38647
>
> So I think that now the TimeController is part of QGIS, and people will start to play with Time-based data more and more (from TimeZone aware datasets like csv and postgres) we will run into issues like above.
>
> Is there something we can do to make QGIS timezone aware?
> OR should we stay far away from it (timezones are hell)...
> OR should we wait with it after QGIS 4

This is a good discussion, and a very complex issue.

Here's some summary points from a recent off-list discussion I had
regarding this very topic:

-  we don't expose time zones anywhere in qgis. while the underlying
QDateTime class has good support for timezone handling, there's
no-where in QGIS you can "see " the time zone for a datetime value,
and there's no functionality present in -the expression engine or
processing to work with time zones/manipulate them.
- different data providers are handling time zones differently, but
most are just giving date time values as a "local time"/"no time zone
available" value. As described above, this inconsistency leads to
issues when you compare values from different sources. Even though the
look the same in QGIS, they aren't!!
- some underlying data sources just don't have any time zone
capabilities, or have limited capabilities. E.g. GDAL has a flag if a
datetime is UTC or "not utc/unknown", and that's it.

I think we have no choice but to start exposing time zone information
to users everywhere we have dates, and move to treating dates without
a timezone as being in "local time" (which may cause issues if
projects + data are shared between users in different time zones). We
should add timezone functionality to the expression engine, and
algorithms for converting time zones to processing. We'd also need to
add a "manual time zone override" option to temporal enabled layers --
so for those sources with ambiguous time zone information the user has
some capacity to manually pick the correct time zone.

It's definitely achievable, but it's not a trivial amount of work. I'd
estimate we're looking at ~5-7 days to do this properly (and it needs
to be done properly, covering all uses of datetime values all
throughout QGIS and the different data providers! Hacking in
one-by-one fixes in individual data providers will just make the
situation worse)...

Nyall






>
> Regards,
>
> Richard Duivenvoorde
>
>
>
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