[root@server ~]# systemctl list-timers fstrim.timer
NEXT LEFT LAST PASSED UNIT ACTIVATES

0 timers listed.
Pass --all to see loaded but inactive timers, too.
[root@server ~]# systemctl enable --now fstrim.timer
Created symlink /etc/systemd/system/timers.target.wants/fstrim.timer → /usr/lib/systemd/system/fstrim.timer.
[root@server ~]# systemctl list-timers fstrim.timer
NEXT                         LEFT        LAST PASSED UNIT         ACTIVATES
Mon 2024-07-01 01:28:50 CEST 4 days left -    -      fstrim.timer fstrim.service

1 timers listed.
Pass --all to see loaded but inactive timers, too.