Expressions like the below should work, but they don't because of a bug that I just found. I'll log this internally to get it fixed in an upcoming beta. You can get the beta if you update to version 15, then download and install the beta.
[DATETIMEFROMSTRING(SUBSTRING([Field8], 1, 10),'YYYY-MM-DD', '-', ':') ]
[DATETIMEFROMSTRING(SUBSTRING([Field28], 1, 19),'YYYY-MM-DD HH:NN:SS', '-', ':') ]
A possible workaround to import your file. Sorry, I know it's a bit of a hassle. If I think of something better, I'll post again.
- Create another table that has fields of all VARCHAR2's.
- Import your data into that
- Run a query to translate and copy that data to the "real" table.