Require full ISO 8601 timestamps

[ivanr]

[Continuing here from https://github.com/w3c/reporting/issues/209]

One of the fields in deprecation reports is "anticipatedRemoval", defined as "A JavaScript Date object (rendered as an ISO 8601 string)". The example contains "2020-01-01" as a possible value.

ISO 8601 supports many different date, time, and timestamp formats. Deprecation Reporting is vague on exactly which formats can be used. From a report processor perspective, it's onerous to handle everything. I suggest picking only one format.

In my opinion, it's best to use the full timestamp, even for this case when full precision is not required. Just zero-out the unnecessary parts.