Once you fix the data on your PC - month, day, year, and time - reboot the PC and OneCare should once again reflect that it is active and good.
If not, open OneCare and click on Check for Updates to see if will refresh the credentials.
Last resort short of contacting support, assuming that your CMOS battery is now good and the date and time will stay correct when you reboot, reinstall OneCare. If the clock resets at each boot, you won't be able to resolve this until you replace the battery.
How to reach support (FAQ) - http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/30400b52-7f26-4ba0-bc18-17e305329d90
-steve
Microsoft MVP Windows Live / Windows Live OneCare, Live Mesh, & MS Security Essentials Forums Moderator