Can credits created today affect Revenue Recognition numbers in the past?

Yes, credits created today can affect earlier time periods if the credit was configured with a Time Range instead of Specific Date.

For example, if I created a credit for $100 today with a time range of January 23-May 23rd and set a Revenue Recognition method of Evenly -> downloaded the Revenue Recognition export for that same time period, I would see that credit update the period of January 23-May 23rd since that $100 amount was recognized daily during that time period.