I don’t remember for sure, but I think the parser don’t recognize that struck. In engine source they are not declared as USTRUCT.
Converting the DateTime to unix integer then saving that int and rebuilding it from the saved int should work.
In the SQLite plugin that’s what I do to save it, a string of int64 is stored then that string is converted to int64 on load and a FDateTime is created from that int64 as return type.