def diff_date(truth_date, computed_date):
    """Compare two dates. Returns (match?, reason for mismatch)."""
    if computed_date == truth_date:
        return (True, '')
    if computed_date is None:
        return (False, 'Missing date')
    if truth_date is None:
        return (False, 'Should be missing date')
    return (False, 'complex')