systemd timers

Timers in systemd are usually harder to create, but easier to handle than cronjobs.

• https://nick.groenen.me/notes/replacing-cronjobs-with-systemd-timers/
• https://trstringer.com/systemd-timer-vs-cronjob/
• https://medium.com/horrible-hacks/using-systemd-as-a-better-cron-a4023eea996d
• https://moshib.in/posts/replacing-cron-jobs-with-systemd-timers/
• https://kofler.info/systemd-timer-als-cron-alternative/
• https://opensource.com/article/20/7/systemd-timers
• https://www.freedesktop.org/software/systemd/man/systemd.time.html