Eleventy date filter applies host timezone instead of preserving UTC offset

Answer

The built‑in Eleventy date filter does not guarantee identical UTC output across different build agents. It creates a Date via new Date(input) (which correctly parses a UTC‑ending string) but then formats it with Intl.DateTimeFormat without supplying a timeZone option, so the output reflects the host machine’s IANA zone.

Confirmed facts

• The behavior is stable across Node versions that support Intl.DateTimeFormat (Node ≥ 0.12) and across operating systems; it depends only on the environment’s locale/zone.
• Pinning an IANA zone in Eleventy configuration does not affect the filter – the filter ignores any global zone setting.

Steps to achieve identical UTC rendering

1. Add a custom filter (or plugin) that calls Intl.DateTimeFormat with { timeZone: 'UTC' } (or use a timezone‑aware library such as Luxon or date‑fns‑tz).
2. Example custom filter (place in .eleventy.js):

module.exports = function(eleventyConfig) {
  eleventyConfig.addFilter('utcDate', function(dateObj) {
    if (!(dateObj instanceof Date)) dateObj = new Date(dateObj);
    return new Intl.DateTimeFormat(undefined, {
      timeZone: 'UTC',
      year: 'numeric',
      month: '2-digit',
      day: '2-digit',
      hour: '2-digit',
      minute: '2-digit',
      second: '2-digit'
    }).format(dateObj);
  });
};

1. Use the filter in your templates: {{ "2026-10-11T01:50:47Z" | utcDate }}.
2. Verify consistency by building the site in two environments with different IANA zones (e.g., UTC and Asia/Tokyo) and confirming the generated HTML is identical.

Missing diagnostic detail

If you cannot add custom filters or plugins, please confirm whether you are able to pre‑process timestamps (e.g., convert them to a fixed offset string) before passing them to the built‑in filter; this would change the recommendation.