In order to free up storage on my iPhone, I want to delete some of my older photos. Instead of deleting each one manually, is there a way to delete photos older than x days, or even delete all the photos from 2017?
Sure - if you don't need to make backups or transfer, install the Shortcuts app (used to be called Workflow) before Apple bought the company and renamed / shipped / enhanced the automation app for iOS.
You need two items - select photos and delete photos
For select, you can choose photos before a date. If you're not sure, you can limit it to 5 photos so you can validate it's getting the ones you want to delete.
Just be sure you know if your iCloud photo library is tied to the iPhone since deleting a photo will then cascade and delete in the cloud as well.