Ionic - Firebase - Automatic functions

Hello, I have a schedule control application made with Ionic 3, and Firebase Real Database database.

When an employee enters, a value is loaded into the database of the time that has to be reported. The report is made by pressing a button of the application and a new brand time is generated (adding 30 minutes).

I need to automatically generate an alert or record in the database when it passes the time you had to report. This alert must be generated even with the application closed. I was looking at Firebase Functions, but I’m not sure which one to use.

I clarify that there are several employee with different hours of entry. The time they have to dial is in the database, within the registry of each officer.

Greetings and thanks.