MTS files downloaded from some Digital Cameras and uploaded to Google Photos (either directly or via Google Drive) don't show up with the correct creation date.

These MTS files often have Date/Time Original metadata set within them.

By taking that Date/Time Original metadata and using it to set the MTS file's modification date you can have Google Photos assign the correct creation time to those files.

Note that this solution relies on exiftool for reading the metadata which can be obtained here: http://www.sno.phy.queensu.ca/~phil/exiftool/index.html