5 minute discrepancy between actual- and displayed UTC offset for moments before "1940-05-15T23:40:00Z" in CET/CEST

[cueedee]

moment.version

'2.24.0'

My system's timezone is CET/CEST which explains the +02:00 UTC offset of:

moment( "1940-05-15T23:40:00Z" ).format( "HH:mm Z" )

'01:40 +02:00'

According to https://www.timeanddate.com/time/change/netherlands/amsterdam?year=1940, on Thursday, 16 May, 00:00 1940 (local time), the UTC offset in that timezone was changed by +01:40, which afaict should mean that moments before 1940-05-15T23:40:00Z, like for instance 1940-05-15T23:39:59Z should display a +00:20 offset.... however:

moment( "1940-05-15T23:39:59Z" ).format( "HH:mm Z" )

'23:59 +00:15'

Would have me to believe that the UTC offset now is +00:15, yet the 23:59 time demonstrates the +00:20 offset that was expected... (23:59 - 00:20 = 23:39 - the UTC time of the argument to moment(...) above)

The above has been observed regardless of Javascript environment tried: NodeJS 10.16.3, Chrome 77.0.3865.120 and FireFox 69.0.3.

[ederius]

I have a related issue, any idea about it?

[cueedee]

@ederius, as can be learned from this issue's creation date it has not managed to attract a great deal of attention since its inception... I have no idea how to proceed.

[ederius]

My issue is easy to reproduce, try with this: moment("1910-12-31").toISOString() the result of that should be 1910-12-31T05:00:00.000Z if your timezone is -05:00, But is 1910-12-31T04:56:16.000Z, subtract 3 minutes. Have you any idea why do that @cueedee?

[cueedee]

@ederius, sorry no, I haven't got a clue. Like you, I'm hoping the good people at Moment.js will be able to explain... If only they could be enticed to have a look...

That being said...

Maybe this will help to provide some insight:

https://www.timeanddate.com/time/map/

Select the city that refers to your timezone, then on the resulting page, select the "Time Zone" menu item, and then in the "Time zone changes for:" dropdown, select the range that includes the year of your interest. Does that list information about any UTC offset changes in that period?

Does it match what you experience in Moment.js, or mismatch?

[marwahaha]

Does this happen on the native Date() object too?