Google calendar API 400 error

Ok, I figured it out and I am leaving the answer for anyone else who is LOSING THEIR MIND over this.

Despite what Google's documentation says about format for datetime in this case, the only format I could get to work was this:

2008-03-07T17:06:02.000Z

so that's YYYY-MM-DDTHH:MM:SS.MMMZ

What is the Z for? I don't know, but it has to be there.