I had no idea what my system code page is, so I searched and found these two commands to display your ANSI (Windows) and OEM (console) code pages:
reg query HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Nls\CodePage /v OEMCP
My result is 437.
reg query HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Nls\CodePage /v ACP
My result is 1252.
The command chcp displays only your OEM code page, and you can change it temporarily to UTF8 (code page 65001):
chcp
Active code page: 437
chcp 65001
Active code page: 65001
I learned that it is not a good idea to change your code pages to UTF8 globally (in the Registry), as it typically makes Windows unbootable with all of its legacy code and dependencies. Furthermore, even changing the console code page to 65001 temporarily can cause problems with certain commands such as find, more, and anything involving piping or redirection.
I looked into the inverted question mark symbol and it is part of extended ASCII (128-255), character # 168.
So, after this crash course on everything I ever wanted to know about code pages, I tried every combination of active code page (437, 1252, or 65001) and one of the following commands (I use the 15th if I don't know the exact date):
-filecreatedate="1972:12:15 14:15:16" -filemodifydate="1972:12:15 14:15:16" -alldates="1972:12:15 14:15:16" -charset filename=437
-filecreatedate="1972:12:15 14:15:16" -filemodifydate="1972:12:15 14:15:16" -alldates="1972:12:15 14:15:16" -charset filename=1252
-filecreatedate="1972:12:15 14:15:16" -filemodifydate="1972:12:15 14:15:16" -alldates="1972:12:15 14:15:16" -charset filename=65001
So, 3 possible active code pages, 3 commands, 9 different combinations, and none of them made the annoying warnings go away.
I guess don't fix what ain't broke. ExifToolGUI changes the timestamps just fine to 1972-12-15, in spite of the filenames 1972-12-¿¿ pic01, 1972-12-¿¿ pic02, 1972-12-¿¿ pic03, and so on generating the weird warning messages.