We use ( new Date() ).getTimezoneOffset() to detect the user timezone on login. This works for 99,999% of the time but sometimes it just return 0 instead of the timezone.

For example, one of our customer has about 500 logins per day, which is about 15k per month. Wrong time zone information occurs about every 3-4 months for one single login, so the error rate is about 1:52500.

They use Chrome ( currently 94.0.4606.81 ) and have a correct system time - when they login again ( after a failed timezone detection ), everything works as intended. It's also always a different user where this problem occurs.

I thought this might be some initialization or implementation problem but couldnt find anything like this online... We have never been able to observe this behavior ourselves, it has always been reported by customers and is also traceable in the logs up to the initialization at login.

Does anyone have any idea what this could be due to or what is the best way to get to the bottom of this ?