My Piwiki installation has 3 sites. Accessing each of them gives the following error message:
Date format must be: YYYY-MM-DD, or 'today' or 'yesterday' or any keyword supported by the strtotime function (see http://php.net/strtotime for more information): now
It worked fine last time I accessed my Piwiki installation.
What is the URL when you get this error? try changign your bookmark or re-set settings in Settings>User settings > Default date to load