The problem here is that you are using Luke to look at the raw data that Solr is putting in the Lucene index. Solr is a layer that sits above Lucene index, and it may choose to encode values however it sees fit.
To see the data in a more appropriate format, I recommend using the Solr web UI. You can access it by visiting the Solr URL in your web browser (something like https://localhost:8983/solr, depending where it is installed), selecting your core on the left in the Core Selector dropdown, selecting Query on the left, and then clicking Execute Query. In this UI, you can also craft Solr queries to select particular data.
Note that in the attached screenshot, my
__smallcreateddate_tdt is showing the expected readable datetime format.