Hey Jone!

> ...
>   (mcron-service-type (mcron-configuration (jobs (list 
>   fstrim-job))))
> ...

This is close, but very slightly wrong. `mcron-service-type` isn't 
a procedure, so you can't call it like that. You have to use 
`service`:

  (service mcron-service-type
           (mcron-configuration (jobs (list fstrim-job))))

The manual instead uses the `mcron-service` procedure, which I can 
see you tried below...

> ...
>   (mcron-service (mcron-configuration (jobs (list fstrim-job))))
> ...

... but in this case, you need to not wrap the jobs in 
`mcron-configuration`. So something like this should work:

  (mcron-service (list fstrim-job))

This is outlined in the manual in (guix) Scheduled Job Execution. 
Were you looking elsewhere in the manual for your examples? If 
there's something that pointed you in the wrong direction, please 
let me know where so I can fix it up.

I hope that helps!

Carlo