I was reading about the I-134 and the guide instructs you to start gathering the required info early. Does it matter when the letter from the bank or employer is dated? I can complete the I-134 form and have it notarized now (I work with a notary) but wasn't sure if the dates had to within a certain timeframe. Any guidance would be appreciated.

I think that if it is all within a couple of months then it will be fine
I got mine all done through a couple of months and a lot of that has to do with waiting anyway. Of course the closer you get these things to the interview date the better.