<How can I keep a countdown timer running even if ...

# stackoverflow

[user]

How can I keep a countdown timer running even if I don't open the app for a long time?
I've been making a timer app using CountDownTimer, and now I'm testing if it works properly. However, if I set a bit longer period, for example 1h30m, and close the app, it stops working, and when I open the app, it shows the initial screen, the timer I set disappears.
I tried to turn off "Battery Optimization" in this app.
I tried to turn on "Background Work" in this app.
But they didn't work.