The Wikidata date parsing code assumes that datetime must end in "Z", whereas this is not a requirement for the standard. Moreover, if the datetime is produced by python's datetime.isoformat(), it will always be in the `2017-08-23T18:34:16+00:00` format, without the 'Z'.
Note that this is a warning, and the date is still stored, but it produces a lot of noise in the logs.
```
20:24:47.328 [com.bigdata.journal.Journal.executorService58564] ERROR c.b.r.internal.LexiconConfiguration IP:127.0.0.1 UA:Jetty/9.2.z-SNAPSHOT - Invalid date format: 2017-0
8-23T18:22:08+00:00: value=2017-08-23T18:22:08+00:00
20:24:47.328 [com.bigdata.journal.Journal.executorService58564] WARN o.w.q.r.b.i.l.WikibaseDateExtension IP:127.0.0.1 UA:Jetty/9.2.z-SNAPSHOT - Couldn't create IV
java.lang.IllegalArgumentException: Invalid date format: 2017-08-23T18:22:41+00:00
at org.wikidata.query.rdf.common.WikibaseDate.fromString(WikibaseDate.java:45) ~[common-0.2.5-SNAPSHOT.jar:na]
at org.wikidata.query.rdf.blazegraph.inline.literal.WikibaseDateExtension.createDelegateIV(WikibaseDateExtension.java:76) ~[blazegraph-0.2.5-SNAPSHOT.jar:na]
at org.wikidata.query.rdf.blazegraph.inline.literal.AbstractMultiTypeExtension.createIV(AbstractMultiTypeExtension.java:73) ~[blazegraph-0.2.5-SNAPSHOT.jar:na]
at com.bigdata.rdf.internal.LexiconConfiguration.createExtensionIV(LexiconConfiguration.java:711) [bigdata-core-2.1.5-SNAPSHOT.jar:na]
at com.bigdata.rdf.internal.LexiconConfiguration.createInlineLiteralIV(LexiconConfiguration.java:660) [bigdata-core-2.1.5-SNAPSHOT.jar:na]
at com.bigdata.rdf.internal.LexiconConfiguration.createInlineIV(LexiconConfiguration.java:521) [bigdata-core-2.1.5-SNAPSHOT.jar:na]
at com.bigdata.rdf.lexicon.LexiconRelation.getInlineIV(LexiconRelation.java:3355) [bigdata-core-2.1.5-SNAPSHOT.jar:na]
at com.bigdata.rdf.lexicon.LexiconRelation.addTerms(LexiconRelation.java:1808) [bigdata-core-2.1.5-SNAPSHOT.jar:na]
at com.bigdata.rdf.rio.StatementBuffer$Batch.addTerms(StatementBuffer.java:1951) [bigdata-core-2.1.5-SNAPSHOT.jar:na]
at com.bigdata.rdf.rio.StatementBuffer$Batch.writeNow(StatementBuffer.java:1881) [bigdata-core-2.1.5-SNAPSHOT.jar:na]
at com.bigdata.rdf.rio.StatementBuffer$Batch.access$1000(StatementBuffer.java:1645) [bigdata-core-2.1.5-SNAPSHOT.jar:na]
at com.bigdata.rdf.rio.StatementBuffer$DrainQueueCallable.drainQueueAndMergeBatches(StatementBuffer.java:888) [bigdata-core-2.1.5-SNAPSHOT.jar:na]
at com.bigdata.rdf.rio.StatementBuffer$DrainQueueCallable.call(StatementBuffer.java:827) [bigdata-core-2.1.5-SNAPSHOT.jar:na]
at com.bigdata.rdf.rio.StatementBuffer$DrainQueueCallable.call(StatementBuffer.java:795) [bigdata-core-2.1.5-SNAPSHOT.jar:na]
at java.util.concurrent.FutureTask.run(java.base@9-internal/FutureTask.java:266) [na:na]
at com.bigdata.util.concurrent.LatchedExecutor$1.run(LatchedExecutor.java:121) [bigdata-core-2.1.5-SNAPSHOT.jar:na]
at java.util.concurrent.ThreadPoolExecutor.runWorker(java.base@9-internal/ThreadPoolExecutor.java:1158) [na:na]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(java.base@9-internal/ThreadPoolExecutor.java:632) [na:na]
at java.lang.Thread.run(java.base@9-internal/Thread.java:804) [na:na]
```