getTimezoneOffset should not round

[SheetJSDev]

• Version: 14.2.0
• Platform: darwin
• Subsystem:

What steps will reproduce the bug?

Run NodeJS REPL in the Asia/Hong_Kong timezone (TZ=Asia/Hong_Kong node) and run

var d=new Date(1899, 11, 30, 0, 0, 0);
console.log(d.getTimezoneOffset())

How often does it reproduce? Is there a required condition?

Always reproduces

What is the expected behavior?

According to https://www.timeanddate.com/time/zone/hong-kong/hong-kong (look at the Time zone changes for: 1850-1899) the timezone was +7:36:42 which would suggest the timezone offset should be -456.7, potentially with a sub-second difference

In case you need an "official" listing, the IANA tz database lists the following for Asia/Hong_Kong:

# Zone	NAME		STDOFF	RULES	FORMAT	[UNTIL]
Zone	Asia/Hong_Kong	7:36:42 -	LMT	1904 Oct 30  0:36:42

So it definitely has a sub-minute component

What do you see instead?

-456 it appears NodeJS is rounding.

According to the spec https://tc39.es/ecma262/#sec-date.prototype.gettimezoneoffset

Return (t - LocalTime(t)) / msPerMinute.

That division is not integer division. Other date functions which do require integer results are explicitly specced with an appropriate integer operation.

Additional information

This also affects Chrome, so this is probably a v8 bug.

Related issues:

• Different date parse result in chrome and firefox. SheetJS/sheetjs#1918
• Different utcOffset() result in firefox and chrome moment/moment#5515

[heroboy]

https://bugs.chromium.org/p/v8/issues/detail?id=7863&q=getTimezoneOffset&can=2

[bnoordhuis]

I was just about to link to that issue. :-)

The relevant code in V8 is this:

node/deps/v8/src/objects/js-objects.cc

Lines 5804 to 5808 in bcdbd57

	if (index == kTimezoneOffset) {
	GetIsolate()->CountUsage(v8::Isolate::kDateGetTimezoneOffset);
	return Smi::FromInt(date_cache->TimezoneOffset(time_ms));
	}

node/deps/v8/src/date/date.h

Lines 95 to 99 in bcdbd57

	int TimezoneOffset(int64_t time_ms) {
	int64_t local_ms = ToLocal(time_ms);
	return static_cast<int>((time_ms - local_ms) / kMsPerMin);
	}

Which operates on ints, not doubles. I'll close this out because it's ultimately not a Node.js issue but we can cherry-pick upstream fixes once they materialize. Thanks anyway for the bug report!