Steps to reproduce the problem:
Can be reproduced in the console (remote debugging) with the following code:
1. Create a new Date some time after a daylight saving time change:
    t = new Date("2017/11/20")
2. Now set the day of the month to some time before the daylight saving change.
    t.setDate(5)
3. Observe the date object

What is the expected behavior?
I expect the time to be the same and only the date to change.

What went wrong?
The timezone is changed:

> t = new Date("2017/11/20")
Mon Nov 20 2017 00:00:00 GMT-0700 (MST)
> t.setDate(5)
1509861600000
> t
Sun Nov 05 2017 00:00:00 GMT-0600 (MDT)

In the above example, the timezone was changed from Mountain Standard Time to Mountain Daylight Time.

I don't know if this is expected behavior if in a time zone that observes DST (it certainly messed up date arithmetic if it is!), but it certainly is a bug in a time zone that does NOT observe DST, as was the case here!

Did this work before? N/A 

Chrome version: 62.0.3202.84  Channel: stable
OS Version: 7.1.2
Flash Version: 

I have tested this code and this bug does not exist in the desktop version of Chrome, e.g. 61.0.3163.100 under Windows 10.