No idea whether FLickr will allow you to change that. I assume that Flickr gets its dates from the exif information in the image - although having just looked at the exif information from one of my scans, the only likely one is "File Modification Date/Time". Perhaps you need change the file modification date, after scanning, and before uploading to Flickr. I use "exiftool" to change these attributes - this command will change that date:
exiftool -FileModifyDate="yyyy:mm:dd hh:mm:ss" file.jpg
This is a very powerful tool for changing image attributes. I use it in command line mode (in a Command window) - not certain whether it has a windows type of front end.
This seems to me to be easier than trying to change the file date in Windows or DOS. I use it regularly when I forget to change my camera's time to a different timezone - all photos in a folder can be adjusted plus or minus an amount. I also use it to change the orientation without having to open the image and save it, in cases where a vertical photo is appearing as a horizontal.