Everything You Need to Know About Date in JavaScript
css-tricks.comThen there's good old IE 11:
foo = new Date(2018, 6, 100); [date] Mon Oct 08 2018 00:00:00 GMT-0400 (Eastern Daylight Time)
What is it about this that you find odd? I just tested it in Chrome and it provides the same answer. I'm pretty sure this is the documented behavior.
Tldr, use moment.js
Can recommend DayJS[0] as a lighter alternative to MomentJS with an almost identical API.