Wow, I have not found that at all. At this point I have had Google Photos Uploader manage to upload photos from the Photos for Mac application via selecting /~Pictures, as well as selecting the "Masters" folder inside the Photos database file. In neither instance did Google take the date that the photo was imported into the OS X app -- it has always respected the original creation date via EXIF metadata. I just tested this, right now, using an old photo from a friend that I had in a separate folder on my laptop. I manually dragged it into Photos. It (correctly) showed up in a previous year (1999) there, and then Google Photos Uploader showed a pop-up that it was uploading, and then when I looked in http://photos.google.com, it was in the correct date. 1999, and not "today."
Aside from this, I feel it's important to mention yet again, that I do NOT have to run the Photos app on my iMac at all, for automatic photo importing/backing up to work. I leave the Photos app quit most of the time. I take photos with my iPhone. iCloud Photo Library syncs regardless, sending the photo from my iPhone to the iCloud library to my Mac, somehow in the background (yes, even with Photos not running) and Google Photos Uploader then sees that new photo and sends it to the Google cloud. Later I launch the Photos app, which shows the photo, too. And I know it's not the Google Photos app on my iPhone that's doing the uploading, because I've had "Back up & sync" turned off there.
Anyway, I wonder if this is a difference between iCloud Photo Library (yes, I am playing $48/year for 200 GB of space) and simple Photostream? In any case, it's all working rather brilliantly. Best of all, if I want to clean up recent photos, deleting bad/blurry ones, and I do this in the Google Photos app on my iPhone, or Google Photos on the Web, and it interacts with iCloud Photo Library. The phone app confirms with me if I want to want to delete these photos from the device and then a second dialog appears asking if I want to delete from iCloud Photo Library. Which means these deletions sync back to iCloud, and Photos for Mac on my iMac and laptop. Which pleases me greatly. It's a bit odd dealing with Google's Photo app when it's Apple I'm actually paying, but this workflow does the job and I feel I'm getting the best of both worlds. I especially appreciate Google's superior contextual search, the auto-awesome creations, and the easier (ecosystem-agnostic) share-ability.
I also wonder why so many people are having so many different experiences with Google Photo Uploader, and how it interacts (or doesn't) with Photos for Mac? All that being said, official support will be nice. But it's working quite well for me at this time. I hope my notes help someone out there!