Yep, it's original.
It actually gives you the option to backup in High quality or Original size.
High quality lets you upload unlimited photos while in original size, you can upload up to 15 GB, and after that, you have to pay for it regularly.
Now the interesting thing is that up to 16 MP, there is really no difference between the image quality, though image size reduces to like 5%.
I think the image details are preserved, but only online. If you download them, then obviously the date of creation be the time when you download the photos.
If accidentally your photos get disorganized, (which is very likely to happen if you move photo data to sd card), you can use a tool called
Photo Date Organizer to fix it.
To avoid this, always transfer data using a PC, and not the phone.