Uses Python packages available on Raspbian GNU/Linux 10 (buster) by default.
To run the script automatically add a line similar to the one below (change paths match your setup) to crontab.
@reboot python3 /code/downtime_monitor.py --data-dir /logs/downtime --heartbeat-interval 30
Please note that if your Rpi is not equipped with an RTC the logged timestamps will be rather chaotic. For example you can get a log entry like
2021-01-01T10:00:10 down between 2021-01-01T10:30:00 and 2021-01-01T10:00:00
Refer to How accurate is Raspberry Pi's timekeeping? for more details.