ISO-8601 Formatted Dates Are Interpreted As UTC
Using new Date() or Date.parse() with a string that represents a date is
a great way to create a Date object for a specified date. A variety of
formats are accepted by these methods.
But, caution!
There are subtle differences in how those dates will be interpreted. Given
any old string that reasonably represents a date, the date will be
interpreted using the local time zone, in my case CST.
> new Date('2017-12-4')
Mon Dec 04 2017 00:00:00 GMT-0600 (CST)
However, as soon as we use an ISO-8601 compliant date format, ECMAScript 5 specifies that the date ought to be interpreted using the UTC time zone. As you can see, the results are drastic enough to affect what day it comes out to.
> new Date('2017-12-04')
Sun Dec 03 2017 18:00:00 GMT-0600 (CST)