It's possible (in a rather arcane way) to change the "Content created" date that PowerPoint reports, but if your students knew how to do that, they'd have changed it to something PRIOR to the due date. It's safe to assume that they don't know the magic.
;-)
But do make sure you're looking at the right property.
Right click the file's icon, choose Properties and go to the Details tab.
The date/time next to "Content created" will be the date and time that the file was first created in PowerPoint.
Don't use the information on the General tab; that will give you the creation information for that particular file on that particular computer. In other words, it may reflect the time you downloaded the file to your PC, saved it from email, copied it from
a thumb drive, etc.
