Hi Phil,

Could be, I mis something, but I would expect, that (for DNG, which contain both, exif and xmp):

exiftool -tagsfromfile src.dng dst.mie
and
exiftool -tagsfromfile src.dng -all:all dst.mie

-would give identical resulting mie file.
Right now, if -tagsfromfile is used without "-all:all", resulting EXIF:ModifyDate has trailing TimeZone (but resulting EXIF:DateTimeOriginal and EXIF:CreateDate doesn't have it).
And if I include "-all:all", then none of resulting EXIF DateTimes has trailing TimeZone.

I'm thinking... As far I have tested, traling TimeZone appears only if:
1. "-tagsfromfile" is used,
2. it's without additional (-all:all) option,
3. it appears only for EXIF:ModifyDate.
-in all other cases (AFAIK), there's no trailing TimeZone result for any of EXIF DateTimes.

This would suggest to strip TimeZone for EXIF:ModifyDate in this case too -especially if source EXIF DateTimes doesn't contain TimeZone trailings. Btw. somehow I have a feeling that resulting EXIF:ModifyDate is copied from XMP:ModifyDate, while other two DateTime values are copied (as expected) from EXIF -that's the only explanation for "from where is trailing TimeZone data comming?".
It's about consistency: when copying metadata, either none of EXIF DateTimes has TimeZone trailing (which I prefer), or all have it (but that's not what most users would expect, though). Even better: resulting EXIF DateTimes should have the same format as source file (when using "-tagsfromfile").
Anyway, as it is, it can be confusing... I'm not saying what you should do -I'm only thinking loud :-)

Greetings,
Bogdan