New Dates Can Take Out Of Bounds Values

You can create a new date by feeding it arguments for year, month, and day.

> new Date(2017, 11, 31)
Sun Dec 31 2017 00:00:00 GMT-0600 (CST)

What happens if we push the day value out of bounds?

> new Date(2017, 11, 32)
Mon Jan 01 2018 00:00:00 GMT-0600 (CST)

It rolls over to the next month.

Does the same happen when we push the month value out of bounds?

> new Date(2017, 12, 31)
Wed Jan 31 2018 00:00:00 GMT-0600 (CST)

Yep.

What about negative values?

> new Date(2018, -1, 31)
Sun Dec 31 2017 00:00:00 GMT-0600 (CST)

It rolls the month, and consequently the year, back.