What version of Roundup and what backend are you using?

I'm having trouble reproducing the behaviour you're seeing. I've added a new test to the Roundup test suite and run it across all four backends with no errors:

    def testDateLeapYear(self):
        nid = self.db.issue.create(title='spam', status='1',
            deadline=date.Date('2008-02-29'))
        self.assertEquals(str(self.db.issue.get(nid, 'deadline')),
            '2008-02-29.00:00:00')
        self.db.issue.set(nid, deadline=date.Date('2008-02-29'))
        self.assertEquals(str(self.db.issue.get(nid, 'deadline')),
            '2008-02-29.00:00:00')
        self.assertEquals(self.db.issue.filter(None, {'deadline': '2008-02-29'}),
            [nid])

Could you please add this code to test/db_test_base.py (near testDateUnset) and run "python ./run_tests.py . testDateLeapYear"