i want to set scheduled task for reboot 3 routers with openwrt, first is wr841hpv3 second wr941v5.1 and third wa901v4 for first i would set every 30 days for second every 20 days and third every 30 days what ic crone cod can you tell me for 20 ? and for 30 days?
can you tell me more specific please
That page is specific. It explains how to use cron and what the syntax is.
That page gives pretty detailed idea from OpenWrt perspective, and for generic cron advice regarding the crontab file format you can read e.g.
https://linux.die.net/man/5/crontab
Or search the internet for crontab advice.