Date.toLocaleString('en-CA') shows different result in SSR

[hyunbinseo]

Describe the bug

new Date().toLocaleDateString('en-CA'); returns M/DD/YYYY in SSR, instead of YYYY-MM-DD.

Reproduction

<script>
  import { onMount } from 'svelte';

  let dateString = new Date().toLocaleDateString('en-CA');

  $: console.log(dateString);

  onMount(() => console.log(dateString));
</script>

# server
4/10/2023

# browser
2023-04-10
2023-04-10

Logs

No response

System Info

System:
  OS: macOS 13.3
  CPU: (8) arm64 Apple M1
  Memory: 53.28 MB / 8.00 GB
  Shell: 5.9 - /bin/zsh
Binaries:
  Node: 18.14.1 - ~/.nvm/versions/node/v18.14.1/bin/node
  npm: 9.3.1 - ~/.nvm/versions/node/v18.14.1/bin/npm
Browsers:
  Chrome: 112.0.5615.49
  Firefox: 110.0.1
  Safari: 16.4
npmPackages:
  @sveltejs/adapter-auto: ^2.0.0 => 2.0.0 
  @sveltejs/kit: ^1.5.0 => 1.15.2 
  svelte: ^3.54.0 => 3.58.0 
  vite: ^4.2.0 => 4.2.1

Severity

serious, but I can work around it

Additional Information

No response

[Conduitry]

This is up to Node, and is going to depend on which locales they ship by default. If you run new Date().toLocaleDateString('en-CA') in the Node REPL, you'll see the same results. I'm not sure how one loads new locales at runtime (as opposed to needing to recompile Node with additional locales built in), but this is unrelated to SvelteKit.

[hyunbinseo]

Node.js returns different result in .toLocaleDateString('en-CA').

v16: yyyy-MM-dd
v18: M/d/yyyy

nodejs/node#46277
nodejs/node#45945

[srl295]

Ideally you should not be depending on the date format outputting a certain format. If you really need a fixed format, toLocale… is not for you.

The locale formats can and do change. That said, this particular change was reverted.