Provide full date/time granularity for date faceting boundaries

[DateTimeFormatter.html#ISO_INSTANT](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/time/format/DateTimeFormatter.html#ISO_INSTANT): YYYY-MM-DDThh:mm:ssZ

e.g. 1972-05-20T17:33:18Z