Execute in background with huge delay on mobile

I’m making mobile game. And I want to send local notification on Android and iOS versions after huge delay. For example after 24 hours or week. It will be great if it works after rebooting device too.
How can I realize it? Thanks!

This is how I would do it:

Store a bool and a float of the exact time-stamp inside your playerprefs. (The bool is so that you only execute the initial code once)
Make a coroutine that runs every 5 minutes or so that compares the current time-stamp to the one you saved initially.