UserAgent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/67.0.3396.87 Safari/537.36

Steps to reproduce the problem:
1.var date = new Date(-25569*86400000) //in Chinese locale time zone.
2.console.log(date.getTimezoneOffset());
Output: -485
3. console.log(date.toString());
Sat Dec 30 1899 08:05:43 GMT+0805 (China Standard Time)

NOTES:
****I know IN 1889, the timezone offset was different from today. 0805 is the correct offset for Asia/Shanghai or Asia/Beijing in 1889.****

but, if the 0805 is the correct offset, why the return hour part is "08:05:43"? shouldn't it be "08:05:00"?

I know in v67, chrome has adjusted the timezone to correct value based on emc262, but how could I get exact timezone offset value with current API? 

This API is used for my OADate system.

PS. I have reported a  bug 851804 , but mark to "Won't fix" without a reasonable explanation. 

What is the expected behavior?
Return exact time zone offset value when timezone is 08:05:43.

Or provide a way to get the exact offset value.

What went wrong?

Did this work before? N/A 

Chrome version: 67.0.3396.87  Channel: stable
OS Version: OS X 10.13.5
Flash Version: