API emissions are timestamped at send time, not measurement time

When emissions are uploaded to the API, the timestamp stored server-side is the moment the HTTP payload was built, not the moment the measurement window ended. The measured timestamp is present in the payload and simply never read.

Impact

A user running a tracker with output_methods=["file", "api"] gets two different timestamps for the same measurement: the CSV row is stamped when the measurement was finalised, the API row when the POST was assembled. The two datasets do not line up, so a CSV row cannot be matched to its API row by time, and a dashboard chart and a local CSV chart of the same run do not agree.

The same defect applies to the run record itself, so a run's start time is also the time its creation request was assembled rather than when tracking started.

Reproduction

Run a tracker with both output_methods=["file", "api"] and compare the CSV timestamp column against the API rows for the same run — they do not agree.

Root cause

EmissionsData.timestamp is set when the measurement is finalised, in codecarbon/emissions_tracker.py:1047:

timestamp=datetime.now().strftime("%Y-%m-%dT%H:%M:%S"),

It survives into HTTPOutput._emit, which passes the whole dataclass down (codecarbon/output_methods/http.py:65-68). But ApiClient.add_emission discards carbon_emission["timestamp"] and generates a fresh one (codecarbon/core/api_client.py:192-193):

emission = EmissionCreate(
    timestamp=get_datetime_with_timezone(),

There is a second site with the same problem: ApiClient._create_run stamps the run the same way at codecarbon/core/api_client.py:234-235.

get_datetime_with_timezone() (codecarbon/core/api_client.py:25-28) is arrow.now() — i.e. "now, at payload-build time".

Expected vs actual

• Expected: the stored timestamp locates the measurement window in time, and matches the CSV row for the same measurement.
• Actual: it records when the POST was assembled.

Fixing this is also a prerequisite for any client-side batching or retry/spill-buffer work, which would otherwise misdate every replayed measurement by the full outage duration.

Start with codecarbon/emissions_tracker.py:1047 and follow the data through codecarbon/output_methods/http.py:65-68 into ApiClient.add_emission in codecarbon/core/api_client.py:192-193. Also inspect ApiClient._create_run at lines 234-235 and get_datetime_with_timezone at lines 25-28. Done means API emission and run timestamps reflect the measured or tracking time and match the corresponding CSV records; verify with the existing test suite.