That's basically what I did. I could do something like
data//=Map[Quantity[#,"Days"]&]
This is what I did:
file = File[
"C:\\Users\\Peter\\OneDrive - Marshall \
University\\Documents\\sql.csv"]
semanticData =
SemanticImport[
file, {"DateTime", "DateTime", "DateTime", "Integer", "Integer",
"Currency", "Integer", "String", "String"}, HeaderLines -> 1]
semanticData //=
MapAt[Quantity[#, "Days"] &, {All, "rental_duration"}]
