Using an offset to calculate Timezone is a wrong approach, and you will always encounter problems. Time zones and daylight saving rules may change on several occasions during a year, and It’s difficult to keep up with changes.
As of April 2022, this works in 93.5% of the browsers used globally.
Old compatibility information
ecma-402/1.0 says that
timeZone may be undefined if not provided to constructor. However, future draft (3.0) fixed that issue by changing to system default timezone.
In this version of the ECMAScript Internationalization API, the
timeZoneproperty will remain undefined if no
provided in the options object provided to the
constructor. However, applications should not rely on this, as future
versions may return a String value identifying the host environment’s
current time zone instead.
in ecma-402/3.0 which is still in a draft it changed to
In this version of the ECMAScript 2015 Internationalization API, the
timeZoneproperty will be the name of the default time zone if no
timeZoneproperty was provided in the options object provided to the
Intl.DateTimeFormatconstructor. The previous version left the
timeZoneproperty undefined in this case.