Systemd » Running a service on a cron-like timer
Create a service, e.g. ~/.config/systemd/user/cron-thing.service:
[Unit]
Description=Do thing
[Service]
ExecStart=/path/to/script.sh
Create a timer, e.g. ~/.config/systemd/user/cron-thing.timer:
[Unit]
Description=Run thing on timer
[Timer]
OnCalendar=*:0/15
[Install]
WantedBy=timers.target
(The calendar format is documented in systemd.time(7))
Enable and start the timer.