New to Home Assistant, please help...

I'm fairly new to Home Assistant and could use some help. I'm trying to create a counter that tracks the number of days that have passed since a specific start date of my choosing. Essentially, I want a days since counter to use in my automations and dashboards.

I'm completely stuck on how to set this up and would appreciate any help. I'm comfortable editing configuration files, and I also have HACS installed if a repository (whatever they're called) is the best solution. I've tried a few approaches but haven't had any luck, so any advice would be greatly appreciated.