DirTaxoWriter sets createTime to null after it put it in the commit data once. But that's wrong because if one calls commit(Map<>) twice, the second time doesn't record the creation time. Also, in the ctor, if an index exists and OpenMode is not CREATE, the creation time property is not read.
I wrote a couple of unit tests that assert this, and modified DirTaxoWriter to always record the creation time (in every commit) – that's the only safe way.
Will upload a patch shortly.