Cron is not part of Fedora Silverblue (as well as Atomic Host).
You can install (overlay) it with rpm-ostree. But it is better to avoid as much as possible to overlay packages in such systems.
So, how to take advantage of systemd instead of cron?

Systemd timers

You can use systemd timers to schedule tasks.

Using sudo

Create a .service and a .timer file

sudo vi /etc/systemd/system/test.service

[Unit]
Description=Print date and test string into /tmp/test

[Service]
Type=oneshot
ExecStart=/usr/bin/sh -c '(/usr/bin/date; /usr/bin/echo test) >> /tmp/test'

sudo vi /etc/systemd/system/test.timer

[Unit]
Description=Run test.service every day at midnight

[Timer]
OnCalendar=Mon..Sun *-*-* 00:00:00

Then start the timer:

sudo systemctl start test.timer

Let’s see the active timers and their status:

systemctl list-timers

You should see your timer:

...
Fri 2019-02-15 00:00:00 CET  23h left      n/a                          n/a          test.timer                   test.service
...

To disable a timer:

systemctl stop test.timer

Timers for unprivileged users

With crontab you can create tasks for an individual user. You can do that with systemd as well, without using sudo.

You should create this directory

mkdir -p .config/systemd/user/

And place here the .service and .timer files described in the previous paragraph.
Then use the --user option.

systemctl --user start test.timer
systemctl --user list-timers

Scheduling syntax

Every 10 minutes (at 0, 10, 20 etc. of each hour):
OnCalendar=*:0/10

Every 2 hours (at 12 am, 2 am, 4 am, etc.): OnCalendar=0/2:00:00

Every 30 minutes starting from when the timer was enabled: OnUnitActiveSec=30m

References

Systemd timers man page: https://www.freedesktop.org/software/systemd/man/systemd.timer.html
Systemd time and date specification: https://www.freedesktop.org/software/systemd/man/systemd.time.html

Update

Update: More about using systemd timers: reboot