Which package?

@formatjs/intl-datetimeformat

Describe the bug

When using the @formatjs/intl-datetimeformat polyfill and loading the locale data for cy (Welsh), the timezone information does not correctly get displayed.

For a user in the Europe/London timezone, dates during the daylight savings period (Mar-Oct) will only show GMT as the timezone. The offset of +1 or BST will not be displayed.

Similarly if the "long" timeZoneName is provided it will output Amser Safonol Greenwich instead of Amser Haf Prydain (what appears in the CLDR json data for cy).

Looking at the generated locale-data which ships with @formatjs/intl-datetimeformat, the cy.js file lists the same content strings for DST times

      "Europe/London": {
        "long": [
          "Amser Safonol Greenwich",
          "Amser Safonol Greenwich"
        ],
        "short": [
          "GMT",
          "GMT"
        ]
      },

Worth noting, the times converted are correct, its just the timezone information which is wrong.

Generally the polyfills have saved me a world of pain in the past and allowed the use of the nicer Intl APIs. Unfortunately now I have some contracts with the teaching branch of the Welsh Government and must ensure they get the correct information.

Any assistance or quick bug-fix would be super appreciated <3

To Reproduce

Codesandbox URL

Codesandbox example of issue

Reproducible Steps/Repo

Simply load up the Codesandbox URL and view the output.

Alternatively:

1. Import datetime polyfill + cy locale-data

import "@formatjs/intl-datetimeformat/polyfill-force";
import "@formatjs/intl-datetimeformat/add-all-tz.js";
import "@formatjs/intl-datetimeformat/locale-data/cy";

1. Set default timezone to Europe/London
   Intl.DateTimeFormat.__setDefaultTimeZone("Europe/London");
2. Display a date in the DST period with timezone information (short or long) in Welsh

new Intl.DateTimeFormat("cy", {
  day: "2-digit",
  month: "short",
  year: "numeric",
  hour: "numeric",
  minute: "2-digit",
  timeZoneName: "short",
}).format(new Date("2025-09-26T12:00:00Z"));

1. Output will not have the correct GMT offset or long text.

Expected behavior

Given the language is set to cy, the user in the Europe/London timezone, and a date is in the DST period,

• when using the "short" timeZoneName, either GMT+1 or BST should be output
• when using the "long" timeZoneName, Amser Haf Prydain should be output (as per CLDR data)

It should mimic environments which don't need the polyfill (built-in Intl support for cy)

• Firefox uses GMT+1
• Safari uses GMT+1
• NodeJs uses GMT+1