If you do not specify a sort order, then the unsorted order to your records will be the order in which the records were created.
I don't know if your report is listing these viewing dates in a portal or if your report layout is based on the viewing dates table. Either way, this is true, but how you sort the records by date will be different.
If your report layout is based on the viewing dates table (this is the most flexible way to set up such a report), simply sort your records by the viewing date field.
If your records are listed ina portal to viewing dates, use the portal sort order setting in Portal Setup... to specify this same order.
thanks that whas it - I had based the report on the "wrong" table, so now I created a new report based on the "right" table, and now it works :)