[idan-friedman]

PR #44908 improved CSV datetime-index export time, but left room for improvement by using pandas' internal batch formatter.

Change

Formatted DatetimeIndex values currently use a per-timestamp loop in DataFrame.to_csv. This change uses the datetime-array CSV formatter already used for datetime columns. It retains scalar formatting for PeriodIndex and for years before 1000 to preserve existing warnings and output.

Measurements and validation

On the original 100,000-row PR #44908 workload, the complete to_csv call measured 469.0 ms before and 107.3 ms after on Linux (4.37×). Four other numeric formats improved 2.75× to 4.07×; a weekday-name format improved only 1.09×. Complete CSV output hashes matched for all six formats.

Focused CSV tests passed on macOS and Linux (141 passed, 10 skipped on each). Local pre-commit checks and git diff --check passed. The full pandas test suite has not run.

AI use

I used Codex with OpenAI gpt-6-sol (xhigh reasoning) to investigate PR #44908 and prepare the implementation, tests, release note, benchmarks, and initial PR text. I used Claude Code with Sonnet 5.5 (high thinking) to review the patch. Its feedback prompted additional format benchmarks and correctness checks. I reviewed and understood every change before submitting.

• closes PERF: DataFrame.to_csv with date_format remains slow for a DatetimeIndex #71843
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions. (N/A: no new arguments, methods, or functions.)
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.
• I have reviewed and followed all the contribution guidelines

Check exactly one of the following, per the automated contributions policy:

• I did not use AI to develop this pull request.
• I used AI to develop this pull request. I prompted it to follow AGENTS.md, I have reviewed and understood every change, and I have described above how I used it and exactly which tool, model version, and effort setting.

[github-actions]

Thanks for the pull request, @idan-friedman! It's linked to #71843, but that issue isn't assigned to you yet. To make sure two people don't unknowingly work on the same thing, we ask contributors to claim an issue first. You can comment /take on #71843 to claim it, but be aware that certain issues cannot be taken if they are labeled e.g. Needs Triage — see the contributing guide for more details.

This pull request is unlikely to be reviewed until #71843 is assigned to you.