I have exactly the same issue on Mac OS 10.6 - FMSA 11. I suppose that Date.php has to be generated somehow by the PHP Site Assistant.
OK I solved the issue - though it is an issue that should be fixed by FMI IMHO.
It turns our that Date.php (and related files) comes with the PEAR classes that are installed with the PHP installation by the FMS installer. You should manually add the Date classes if you use your own installation of PHP. This should fix the issue.