- add a Date() entry to an issue in the classic tracker
- add an input field for it in issue.item.html
- create a new issue, type "xxxx" into that field
- submit

Results in the traceback attached. The input check works
when modifying an existing item. To fix it, I added a check
against str into DateHTMLProperty.field:

        if self._value is None:
            value = ''
        elif type(self._value) is type(''):
            value = self._value
        else:
            tz = self._db.getUserTimezone()
            value = cgi.escape(str(self._value.local(tz)))

Now it works nicely. Roundup version is 0.7.5.