i want to make a phone silent on selected time how to do that

Discuss
1

how to make mobile silent at the time i have selected

2

Any specific reason for this

3

Try the settings extension App Inventor Extensions: Settings | Pura Vida Apps to set the volume of alarm, ringtone and music to 0
There are also other extensions available to do this
Get this running on button click first

The alarmmanager extension is able to do something at a given time App Inventor Extensions: Alarm Manager | Pura Vida Apps

Taifun