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Hi Scott,
If you uploaded the file to SharePoint Online, that action will be considered as the creating. Date Modified column will take the action as the last one and show it in the metadata, so the the Midified Date cannot be changed.
We suggest you share your idea in SharePoint UserVoice. That's the best way to tell the team how to imrove SharePoint for you. Here are some ideas related to this topic. You could vote for them and share your comments.
If it is acceptable, you could add a column to the library, and use it to show the last modified data, and manually add the time to it. SharePoint has a default column you could add it fromLibrary Settings > Add from existing site columns >Core Document Columns Group > Date Modified
Then you could edit the date manually.
Regards,
Alex
[Updated by Alex Wang MSFT Oct-03, 2018 11:08 PM UTC]