Cube brush on a time-zone-aware source (not UTC) commits its edges in the wrong zone

[jvdd]

Problem

A cube brush on a histogram over a time-zone-aware Datetime column whose zone is not UTC keeps the wrong rows.

The client commits each edge as a UTC wall-clock string without an offset: fvPhysicalToTemporal in runtime/cube.js formats the physical value with getUTC*. The server reads a string without an offset on a zone-aware column as wall-clock time in the column's zone (_typed_temporal_lit in trace/base.py). So each bound moves by the zone's UTC offset, which is 1 or 2 hours for Europe/Brussels. The committed selection then keeps other rows than the bars that the user brushed.

Reproduction

At the predicate level, on fix/cube-source-grid (PR #134). main builds the strings the same way.

import datetime as dt

import polars as pl

from flexviz.predicates import predicates_to_expr
from flexviz.spec import SelectionPredicate

row = dt.datetime(2024, 8, 2, 6, 4, 2, 441000, tzinfo=dt.timezone.utc)
# What the cube commits for the bar that holds the row: UTC wall-clock time.
rng = ["2024-08-02 06:04:02.441000", "2024-08-02 06:04:02.455000"]
for tz in ("UTC", "Europe/Brussels"):
    df = pl.DataFrame({"t": pl.Series([row]).dt.convert_time_zone(tz)})
    clause = {"column": "t", "range": rng, "closed": "left"}
    pred = SelectionPredicate.model_validate({"clauses": [clause]})
    print(tz, df.filter(predicates_to_expr([pred], df.schema)).height)
# UTC 1
# Europe/Brussels 0

A bound that falls in the spring daylight-saving gap of the zone, for example 2024-03-31 02:30:00 for Europe/Brussels, raises a ComputeError when the query is collected: "datetime '2024-03-31 02:30:00' is non-existent in time zone 'Europe/Brussels'".

Scope

• This predates PR fix(cube): brush binned sources on their own bin grid #134.
• Since PR fix(cube): brush binned sources on their own bin grid #134, the committed strings are also the stored selection box. A fix must keep a box format that Plotly can parse.
• The tests cover only naive and UTC columns. It is not known yet how the display and the non-cube selection path handle a zone-aware column: in which zone the bars are drawn, and in which zone Plotly's selection strings are.

Fix direction

Find out in which zone the display draws a zone-aware column first. Then pick one:

1. Send each edge with an explicit offset (Z). _typed_temporal_lit already converts an offset string to the UTC instant on a zone-aware column, and keeps the wall-clock time on a naive column. The box needs a separate string that Plotly can parse.
2. Send the column's zone in the cube header, and format the strings as wall-clock time in that zone, as the display shows them. Bounds in a daylight-saving gap or overlap still need a rule.
3. Refuse a cube for a zone-aware column whose zone is not UTC, as for Datetime("ns"), so the brush falls back to the server path.

Found in the review of PR #134.