Hey there Ben,
Thanks for stopping by the Apple Support Communities! It sounds like you're trying to free up some space on your iPhone by removing all pictures and videos, and you are still seeing 23.4 GB taken up by those files. You've already made sure to empty your Recently Deleted album, set the date back on your phone, and performed a hard reset on your device. You've done some great work thus far, and I'll be happy to provide you with some ideas to move forward from here.
Prior to diving into any other troubleshooting, it would be beneficial to make sure you have a recent backup of your iPhone available. This will help us ensure that all your important data is stored safely for you. Here's an article with information on how to back up your device:
How to back up your iPhone, iPad, and iPod touch
Once that's in place, I'd recommend resetting the settings on your phone. This won't impact any of your personal data, but you may need to adjust specific user settings, as well as re-enter Wi-Fi networks and passwords. To complete this step, go to Settings>General>Reset>Reset All Settings. When your phone restarts, check to see if the local storage on your phone has updated to reflect the changes you've made related to your pictures and videos.
Beyond that, you would want to look at erasing your phone and resetting it to factory settings. It's possible that some sort of software corruption is causing the storage discrepancy you are experiencing. Setting up your phone as a new device through iTunes would install a fresh copy of iOS, and also rule out any corrupted files, etc. within your user data. Here's a resource for that process:
Use iTunes on your Mac or PC to restore your iPhone, iPad, or iPod to factory settings
For future reference, it may be helpful to utilize iCloud Photo Library to upload and store your photos. There is a feature that allows you to optimize storage on your iPhone so that your pictures and videos do not take up as much local storage space. Check out this resource to learn more about iCloud Photo Library:
iCloud Photo Library FAQ
Have a fantastic weekend!
