Mike if you just want the Date then use this:
DATE( LEFT([Date], LEN(LEFT([Date], FIND([Date], ',' )))+5) )
If you need Date Time it'll be a bit more gnarly. Word of warning: doing string manipulation in Tableau can kill performance on larger data sets.
One positive caveat to the performance warning: If you use an extract, a calc like this will be materialized in the extract so it will only be computed at extract creation/update time and usage of the field will be very fast.
One more thing: in Tableau v8.1, there's a new DATEPARSE() function that will remove the need for string manipulation, however I have no idea of how fast it will run.