Hi Michael
You can definitely set rewards by product. However, there is currently no ability to set the reward effective date range. If you have a lot of products to set, you can perhaps export out the Product Variants and make the change to the Reward points field and re-import quickly on those dates.
Once a reward is earned, the customer can spend it any time in the future even if the reward is turned off for the product.
I hope that helps. Thank you