Hey
i had same problem, which occur if you try to import (add item) CSV to arcgis online. When the pop up window show everything is ok, but its not. As Mark pointing to fact that "Date" field has "date" field type and everything seems fine, but if you finnish import, field type just switch from "date" to "string".
It's caused by format of date. You have to keep supported format: MM/DD/YYYY hh:mm:ss.
Just open CSV in exel, change format of cell to supported format, save and import to arcgis online. If you can't find exact MM/DD/YYYY don't worry, format MM.DD.YYYY worked for me either
Flies away.....