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time: js/wasm sets time.Local to a UTC offset instead of the actual timezone #44408

Splizard opened this issue Feb 19, 2021 · 2 comments


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@Splizard Splizard commented Feb 19, 2021

In time/zoneinfo_js.go the local timezone is determined by the UTC offset returned by the JS call:

(new Date()).getTimezoneOffset()

However UTC offset != timezone and this behaviour causes issues with displaying 'daylight savings' times. This behaviour is unexpected and undocumented and I would expect that time.Local is set to the JS environment's timezone (where available) instead of the current UTC offset.

The timezone can be determined in JS with the Internationalization API:


As long as the tzdata package has been imported/embedded and the its init function runs first, adding the following code to the beginning of initLocal should (in theory) fix this issue.

func initLocal() {
	if intl := js.Global().Get("Intl"); !intl.IsUndefined() {
		var err error
		tz := js.Global().Get("Intl").Call("DateTimeFormat").Call("resolvedOptions").Get("timeZone").String()
		Local, err = LoadLocation(tz)
		if err == nil {
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@agnivade agnivade commented Feb 28, 2021

The internationalization API was the initial approach that was used. But then there was a bit of back and forth in the CL and it was decided to parse the timezone offset from new Date().

cc @neelance

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@neelance neelance commented Feb 28, 2021

I think the real issue here is that there was no yet, so the wasm binary never shipped with any timezone data included. Calling LoadLocation simply did not work on wasm.

Now that 6d63a74 added time/tzdata we could reconsider this.

fanaticscripter added a commit to fanaticscripter/EggContractor that referenced this issue Mar 19, 2021
… date...

due to DST.

This is a bug in golang's wasm runtime itself:
golang/go#44408, so we instead move the datetime
handling to JS. And we introduce dayjs since even formatting a date as
YYYY-MM-DD is a royal pain in JS, even in 2021...
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