When trying to add a reference to a wikidata claim (via some hand-crafted js hitting the API endpoint directly) I get the error:
{"error":{"code":"modification-failed","info":"Data value corrupt: $timestamp must resemble ISO 8601, given +2020-03-07T00:00:00.000Z","messages":[{"name":"wikibase-validator-bad-value","parameters":["$timestamp must resemble ISO 8601, given +2020-03-07T00:00:00.000Z"],"html":{"*":"Data value corrupt: $timestamp must resemble ISO 8601, given +2020-03-07T00:00:00.000Z"}}],"*":"See https://www.wikidata.org/w/api.php for API usage. Subscribe to the mediawiki-api-announce mailing list at <https://lists.wikimedia.org/mailman/listinfo/mediawiki-api-announce> for notice of API deprecations and breaking changes."},"servedby":"mw1358"}But +2020-03-07T00:00:00.000Z is a valid ISO 8601 timestamp (minus the "+" sign).
When I strip the ".000" from the date string then the request succeeds.