When the video was converted from .m2ts to .mp4 format, there would have been some compression. When you post the video, you want to ensure your settings are set to “original quality”. However, I do see some information that indicates they may still convert the video which may result in a loss of quality. You could always backup the original video, even in the .m2ts format, to Google Drive instead of photos.
The metadata, including the “taken date”, may have been lost in the original conversion. However, Google photos does not have a sort feature that allows you to sort by date or otherwise. They are counting on you using the search feature to find photos, so make sure to title and tag your pictures. The best way to organize them by time, would be to create albums.
Hope that helps!