Time Zone Unchanged After Applying Shift

[ilovethemets]

I'm using the Windows shortcut to execute Exiftool.
Here's the command I'm using: C:\exiftool\exiftool(-k).exe -v3 -"TimeZone"+="-8"

I'm trying to shift the Time Zone. I've been at it for days. The documentation indicates that this can be done, but I can't get it to shift, for the life of me.
I've copied a portion of the output generated by Exiftool. I've also included an edited copy of the MP4's metadata.



Shifting Canon:TimeZone if tag exists
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======== F:/Photographs/2024/20240420 Testing with EXIFtool & Videos/MVI_5245 - Copy.MP4
Rewriting F:/Photographs/2024/20240420 Testing with EXIFtool & Videos/MVI_5245 - Copy.MP4...
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Nothing changed in F:/Photographs/2024/20240420 Testing with EXIFtool & Videos/MVI_5245 - Copy.MP4
    0 image files updated
    1 image files unchanged



---- ExifTool ----
ExifToolVersion                 : 12.77
Warning                         : [minor] Missing 2 entries in CanonCustom2 group 4 directory
Warning                         : Possibly corrupted CanonCustom2 data
---- System ----
FileName                        : MVI_5245 - Copy.MP4
Directory                       : F:/Photographs/2024/20240420 Testing with EXIFtool & Videos
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---- Canon ----
CanonModelID                    : PowerShot G9 X Mark II
TimeZone                        : -04:00
TimeZoneCity                    : (not set)
DaylightSavings                 : On

[Phil Harvey]

In general, maker note metadata in MP4 videos is not directly editable by ExifTool.  For Canon, this metadata is likely stored in the ThumbnailImage.  You can extract this image, edit the metadata, then write it back again.  See this post for the technique.

- Phil

[StarGeek]

Additionally, very few programs will read MakerNotes and I don't think there are any at all that will read them in a video. There's no real need to change it, as nothing except running exiftool on it will see it.