There are two possible reasons for this.
1. Your PC date and time are incorrect
2. They have changed the "timebomb" in the beta code because we are very close to release.
I think that the latter is likely, though I can't confirm. I would not worry about it as a production release is indeed expected very shortly - within your 15 day countdown. Once the final is released, you will need to uninstall the beta and install the final, which will be a 90 day trial and you can subscribe/activate with your retail key or LiveID if you already have a subscription active.
-steve