Inconsistent return value of valueOf() on differnt NodeJS versions

[StefanPavlik]

Bug description
The return value of Moment.valueOf() is not consistent with different versions of NodeJS.
It seems that the problem started since NodeJS version 10.4.0

To reproduce
Run following code with different NodeJS verions:

for (let i = 1800; i < 2000; i++) {
  const dateStr = i + '-01-01';
  console.log(moment(dateStr).valueOf() + '    ' + dateStr);
}

Expected behavior
I think that return value of Moment.valueOf() should be the same regardless of NodeJS version.

Results from my computer:
moment.valueOf_on_node_8.16.1.txt
moment.valueOf_on_node_10.3.0.txt
moment.valueOf_on_node_10.4.0.txt
moment.valueOf_on_node_10.16.3.txt

Environment:

• OS: Windows 10 1809 (64bit)
• NodeJS (versions 8.16.1; 10.3.0; 10.4.0; 10.16.3)

Moment-specific environment:

console.log((new Date()).toString())
console.log((new Date()).toLocaleString())
console.log((new Date()).getTimezoneOffset())
console.log(moment.version)

With NodeJS 8.16.1 the output is:

Fri Aug 23 2019 10:05:12 GMT+0200 (Central Europe Daylight Time)
2019-8-23 10:05:12
-120
2.24.0

With NodeJS 10.16.3 the output is:

Fri Aug 23 2019 10:05:26 GMT+0200 (Central European Summer Time)
8/23/2019, 10:05:26 AM
-120
2.24.0

[ashsearle]

Confirmed.

One reason for the difference across Node versions is their every-increasing support for historic timezone data.

Browser/NodeJS parses ISO dates as though they represent midnight at the start of the day UTC.

new Date('1800-01-01T00:00:00.000Z').toISOString() // 1800-01-01T00:00:00.000Z 
new Date('1800-01-01').toISOString(); // 1800-01-01T00:00:00.000Z

moment's custom ISO parsing behaves differently:

moment('1800-01-01T00:00:00.000Z').toISOString(); // 1800-01-01T00:00:00.000Z
moment('1800-01-01').toISOString(); // 1800-01-01T00:01:15.000Z !! (Chrome 76)

I think moment originally implemented its own ISO parsing to support IE, but guess this is an unwanted consequence, and could be avoided if moment feature-detected and used built-in ISO-parsing support where available.

[StefanPavlik]

To achieve the desired behavior one can use the moment.uct() constructor:

for (let i = 1800; i < 2000; i++) {
  const dateStr = i + '-01-01';
  console.log(moment.utc(dateStr).valueOf() + '    ' + dateStr);
}

Using the moment.utc() constructor the results of .valueOf() are the same across different Node.js versions.