Reminder after period

Hi.
I was wondering how I can have the app to make a reminder after ex. 2 weeks. Even if the app is closed. Hope someone have a good idea or is it not possible at all???
Be.

You can use onesignal!

Use could try and use firebase to store the last time the app was logged in. You would need a “host app” that could check each when each user was last logged in and if >2weeks then send a OneSignal message to that users ID. Im not 100% positive it would work but it might.

Thanks. I was hopping for somenthing could work ON the phone. But maybe I cant get arround to use a DB or onesignal.
If I use one signal do I also have to use the calender to trigger the onesignal or can I program that in onesignal you think??
Is there other ideas out there please???