When you give a time format like: "+2017-03-08T12:11:02Z", you get the following error message:
> {'error': {'messages': [{'html': {'*': 'Malformed input: +2017-03-08T12:11:02Z'}, 'parameters': ['+2017-03-08T12:11:02Z'], 'name': 'wikibase-validator-malformed-value'}], 'info': 'Malformed input: +2017-03-08T12:11:02Z', 'code': 'modification-failed', '*': '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': 'mw1343'}
which doesn't actually tell you what is wrong.
After several excruciating minutes, I figured out it wanted "+2017-03-08T00:00:00Z"
It would be helpful if the error message was clearer :D
This should also be in here: https://www.wikidata.org/wiki/Help:Dates