If you want to change the date for a run, or duplicate a run to add it to a different day, you can do this with a tool called FitFileTools.com.

Export the FIT file from PowerCenter, and import the file into FitFileTools.com. There is a tool called Time Adjuster which allows you to set the start time and date of your run. After you change the date of your run, download it to your computer and import it again into PowerCenter.