Unable to confirm security token

I am getting this message with every login. What can I do to fix it.

Unable to confirm security token. A likely cause for this is that some time passed between when you first entered the page and when you submitted a form or clicked a link. If that is the case and you would like to continue with your action, please click the Confirm button. Otherwise, you should click the Cancel button to return to where you were.

After getting the message things seem OK. It happens from all accounts and on all computers logging in.

Thanks,
Bill