Hi Mike,
My pleasure to assist you.
As per your description, I have tested from my side in shared Excel workbook online and it work as expected.
Based on my test if you cell or rang of cell format cell into date it will show incorrect value as you mention. I suggest you please try to select cell or range of cell and right-click the cell chose
“Number Format” in drop down menu and select “General” it should be pasted correct value.

Please give a shot on following methods and see the result.
1: Switch to a different browser and open Excel workbook. Microsoft Edge is recommended.
2: Clear browser’s data. For Microsoft Edge, you can Click “. . .” > Setting > Clear browser data.
In addition, please also check the Keyboard Shortcuts settings for Excel for Web and see whether the right shortcuts have been assigned. You can go to
Help menu >
Keyboard Shortcuts to verify it. For your reference:
Control keyboard shortcuts in Excel for the web by overriding browser keyboard shortcuts

If issue still persist,
may I know from you at your convenience does this problem only occur in this workbook opened in online? You can also try to create a new workbook and open it in online and see whether the shortcuts work properly.
I appreciate your patience and understanding.
Best Regards
Waqas Muhammad