As Mike has stated, you need to upgrade your xenForo version. I presume you are running 1.2.3? (or below).
We had a similar issue, your Cron isn't running and is stuck in a time that has already passed, this issue was fixed/tweaked in xenForo 1.2.4, and we haven't faced it after updating to 1.2.4
Edit: In the case of you not being able to update anytime soon, just "disable" one of the Cron Entries, wait for a couple of seconds, enable it again and refresh your page. Cron Entries should then run on the next correct time. Updating asap is better!