Assuming you're using Windows, part of the problem is that there is no "Date Taken" tag. Windows fills this property with different metadata, depending upon what it can find. The mostly likely tag is the EXIF:DateTimeOriginal
tag. So the best command for you to try would be:
ExifTool "-FileModifyDate<EXIF:DateTimeOriginal" DIR
Edit: I made a more complete command. This will set the FileModifyDate
by trying all the various metadata Windows uses for the "Date Taken" property. It uses the fact that when ExifTool has two assignments that affect the same tag, the latter takes precedence.
ExifTool "-FileModifyDate<XMP:DateTimeOriginal" \\
"-FileModifyDate<EXIF:CreateDate" \\
"-FileModifyDate<XMP:CreateDate" \\
"-FileModifyDate<$IPTC:DateCreated $IPTC:TimeCreated" \\
"-FileModifyDate<EXIF:DateTimeOriginal" DIR
exiftool -time:all -a -G0:1 -s yourfile.jpg
and give the exact "date modified" tag (and group) you want to change? \$\endgroup\$