Scheduling tasks with systemd timers contains an example configuration for scheduling in CoreOS. See systemd.timer — Timer unit configuration for more information on timers. See systemd.time — Time and date specifications for time and date specifications. See Getting started with systemd for examples using systemd.
/etc/systemd/system
.timer
suffix in /etc/systemd/system
systemctl status xxx.timer
systemctl enable xxx.timer
systemctl list-timers
/etc/systemd/system/update_cert.service
[Unit]
Description=Renew Let's Encrypt Certificate
[Service]
Type=oneshot
ExecStart=/bin/bash -c "/home/core/scripts/update_cert"
/etc/systemd/system/update_cert.timer
[Unit]
Description=Update Let's Encrypt Certificate every 3 months
[Timer]
OnCalendar=*-02,05,08,11-01 00:00:00 America/New_York
[Install]
WantedBy=multi-user.target