Check published/updated datetimes for ISO 8601 compliance

With the usual markup demos, how to do human-readable + machine readable, maybe even PHP/Python/etc. demo formatting strings?

Could also try to parse the human-readable date with `new DateTime()` and print the ISO8601-compliant equivalent if it parses successfully.

Review the existing markup demos and their published/updated datetime handling first. Define the accepted ISO 8601 output and the human-readable parsing behavior, then verify that each demo produces compliant machine-readable dates; the issue names no files or tests.

Written by the indexing model from the issue text.