GitHub user gianfranco-campana-vs created a discussion: Is local! the intended
mechanism for local-time D3 formatting?
I'm investigating timezone formatting in Apache Superset and would like to
clarify the intended behavior of the D3 time formatter.
Superset officially uses UTC for timestamp handling, which I understand and am
not questioning.
However, while investigating the frontend formatter used by the Table Chart, I
found that Superset has an explicit mechanism to switch between UTC and
local-time formatting.
For a temporal column, using this D3 format:
%H:%M
the Table Chart formats the timestamp in UTC.
Using:
local!%H:%M
the same timestamp is formatted using the browser/local timezone.
For example, with Europe/Rome during CEST:
%H:%M → 10:30
local!%H:%M → 12:30
I traced this behavior through the frontend code.
The relevant formatter logic recognizes the `local!` prefix and converts it
into `useLocalTime: true`. The formatter construction then uses local
formatting when `useLocalTime` is true and UTC formatting otherwise.
In other words, the code path appears to be explicitly designed to distinguish:
useLocalTime = false → utcFormat()
useLocalTime = true → format()
and `local!` is the mechanism used by the formatter to request the latter.
What I cannot determine is the intended user-facing status of this mechanism.
In the Table Chart UI, the "D3 format" selector provides predefined formats
such as:
%Y-%m-%d %H:%M:%S
%d-%m-%Y %H:%M:%S
%d.%m.%Y
%d/%m/%Y
%H:%M:%S
%Y-%m-%d
Adaptive formatting
but there is no indication that prefixing a format with `local!` changes the
timezone behavior.
For example, there is no visible option such as:
local!%Y-%m-%d %H:%M:%S
I also could not find documentation explaining the `local!` prefix or how users
are expected to request local-time D3 formatting.
This is particularly important for BI dashboards used in organizations
operating outside UTC.
For users working in a local timezone, timestamps displayed in dashboards are
not merely a formatting preference: they are part of the meaning of the data. A
timestamp displayed two hours earlier or later can affect how users interpret
events, sequences, durations, operational activity, and ultimately the business
decisions based on the dashboard.
Therefore, the ability to explicitly request local-time rendering is an
important aspect of making timestamp visualization usable in real-world BI
scenarios, even when UTC remains the canonical timezone used internally by
Superset.
My question is therefore:
**Is `local!` the intended and supported mechanism for displaying D3-formatted
timestamps in the user's local timezone?**
If yes, is there a reason why this mechanism is not documented and/or exposed
in the D3 format choices of the Table Chart?
I am particularly interested in understanding whether this is an established
Superset feature, an undocumented feature, or functionality that is still under
development.
I can provide the relevant frontend code path and a more detailed description
of the debugging process if useful.
GitHub link: https://github.com/apache/superset/discussions/43551
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