photoTakenTime defines when the photo was taken, and should be used for naming the copied output file and directory.
The value that was previously used, creationTime, defines when the source photo/video hit the Google Photos service. This may be very close to photoTakenTime if the device is able to upload the file immediately. Or it may be substantially different if the file could not be promptly uploaded (e.g. no wifi) or if the photo/video timestamp was edited in the Google Photos web UI.
photoTakenTime defines when the photo was taken, and should be used for naming the copied output file and directory.
The value that was previously used, creationTime, defines when the source photo/video hit the Google Photos service. This may be very close to photoTakenTime if the device is able to upload the file immediately. Or it may be substantially different if the file could not be promptly uploaded (e.g. no wifi) or if the photo/video timestamp was edited in the Google Photos web UI.