Only an ISO 8601 date or date-time ts counts as dated; Date alone parses
strings such as "junk 1" as real days, which let junk rows escape the
bound and inflate days_span. A zoneless time, T- or space-separated, is
read as UTC. Tests cover array-valued fields, an offset ts in days_span,
the space separator and every undated row reaching the shared bucket.
Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>