Actually, nothing is done on upload/import, but X3 reads
EXIF dateTimeOriginal, stored inside images where this data was written by cameras (and hasn't been erased). This means that X3 will assign image dates in this order:
- From EXIF dateTimeOriginal, if this value exits in the image.
- Else it will use server MTIME (the date the file was created or modified on server).
You can't actually "edit" the date in X3. There are some apps that can do this, for example
exiftool.org, which runs from the command-line. Optionally, you would have to choose "custom" sort order, and drag items into correct order instead.