There's nothing wrong with the sentence.
I'll split the sentence into two parts,
- You must redeem your free Psychonauts key by 10AM.
- You must redeem your free Psychonauts key by September 30th, 2017!
Two distinctly clear sentences, no confusions whatsoever.
Now the tricky part is when both of them are combined and when the punctuation comes into play.
The comma (,) is what brings about the maximum confusion in interpretation, for many people.
To bring about more clarity in the sentence, I'll restructure it as:
You must redeem your free Psychonauts key by 10AM, September 30th, which is a Saturday.
Now, what do I get from this sentence? September 30th, which is a Saturday, is the last day to redeem the free ticket, and I have to get it before 10AM, the very same day.
There are many ways to restructure the sentence, but the original sentence is perfect.
Try reading it out loud:
"You must redeem your free Psychonauts key by 10AM Pacific" (break) "on Saturday," (break) "September 30th, 2017!"