It seems the clock app shipped with Zenfone 6 does not include the "Silence after" option that pretty much every other Android phone does and hence, when a clock alarm is set, unless the user manually stops the alarm, the alarm will continue indefinitely and never stops on its own (until the phone's battery runs out and the phone turns off, I guess). Is there an option somewhere in the phone/clock app settings that can control the duration of the alarm that I am missing? If not, I would say this is a very important feature that could be added in future versions of the firmware.
P.S. I am using the stock Android 9.