When DateTimeOriginal is absent...

[marciano]

I have some photos which lack Date Time Original metadata. 
exiftool -FileModifyDate /path-to/IMG.jpg

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File Modification Date/Time     : 2017:12:22 18:12:27-03:00    (example)
that seems it is the original date despite their tag name
I want to add this date to DateTimeOriginal so I run
exiftool -DateTimeOriginal=FileModifyDate /path-to/IMG.jpg 

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Warning: Invalid date/time (use YYYY:mm:dd HH:MM:SS[.ss][+/-HH:MM|Z]) in ExifIFD:DateTimeOriginal (PrintConvInv)
What am I missing?
Thank you

[StarGeek]

See Common Mistake #5c.

[marciano]

Hi.
Mac 10.14. ET ver 11.23
I already had used "<" instead of "=" before posting but the source tag is treated as a file
-bash: FileModifyDate: No such file or directory

[StarGeek]

See 4th paragraph under the -TagsFromFile option

    Note that this argument must be quoted to prevent shell redirection,

So try
exiftool "-DateTimeOriginal<FileModifyDate" /path-to/IMG.jpg
if on windows, or
exiftool '-DateTimeOriginal<FileModifyDate' /path-to/IMG.jpg
if on mac/linux

[marciano]

Thanks for your quick reply.
I have a list of commands I tried and one of them was (I copy-paste from Mac TextEdit)
exiftool '-DateTimeOriginal<FileModifydate' ...
that didn't work.
Now I copy-paste your command
exiftool '-DateTimeOriginal<FileModifyDate' /path-to/IMG.jpg

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1 image files updated
and it does the job.
I see both single quotes are not the same in my textarea while writing this post, also noticed in Preview
I use single quotes all the time.
I attached two screenshots of my text area.
Thank you

[StarGeek]

Word processors and things like Google docs will "help" you out by replacing regular single/double quotes with fancy or smart quotes.  There's also a setting on the Mac to do this, apparently universally.  Take a look at this Superuser answer for more info.