You must add a date to this periodical to limit the index, following the format used in NameBase. For all but a few periodicals, this is yyyy-mm-dd with leading zeros as necessary. At a minimum, the yyyy is required. If information after yyyy is included, the index will be limited to a subset within that year. This could be a single month (yyyy-mm), a 10-day period (yyyy-mm-d), or a single day (yyyy-mm-dd).