I am working to determine the orbit of a satellite based on a long exposure photograph, and need precise observations.
Answer
The EXIF standard describes the DateTimeOriginal tag simply as "the date and time when the original image data was generated." It gives no guidance about what event (e.g., shutter released, shutter open, shutter closed, sensor read, post processing) should be used to determine the value.
As mentioned in a comment, the best way to find out how your camera defines it is through experimentation. Synchronize its clock to one you can observe, record the start and end times of a long exposure and see which one ends up in the EXIF. Note that the result you get will only be valid for that one camera with whatever firmware it was running.
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