To execute a command at 13.00 hours
$ at 13.00

at> df -Th > df.txt
at> CTRL+D

The command $ df -Th will executed and stored in df.txt at 13.00 hours

To list the jobs in queue with job numbers
$ atq

To cancel the jobs in queue
$ atrm <job_number>

To execute a command at 14.00 hours
$ at 14.00
at> free -h > free.txt
at> CTRL+D
The command $ free -h will executed and stored in free.txt at 14.00 hours

check with 

$ atq