Any genealogist can testify that dates in family research are anything but rock solid.
Not only do we often find dates that are inaccurate, but we often have only a portion of a full date. In addition to maintaining dates for us to see and use in our research, Reunion uses specifically-formatted dates to sort events and calculate ages and other dates. Please refer to the electronic manual for some other examples of valid approximation entries. The question mark will always move to the beginning of the date entry, no matter where you enter it. In my own family file, I enter a question mark in the death date field of people I know must be dead, but for whom I have no death date.