How to create an executable script that starts execution at a specific date and time?
 

How can I create an executable script that starts execution at a specific date and time (if is possible without rebooting the PC)?

You can use the "at" command to schedule a one-time execution at an arbitrary date and time, or use cron to schedule a repeating execution.

Create the script.
Locate it in some directory, such as /usr/local/bin
Add a crontab entry for the correct user: root's if root privs are needed

BTW, the default env inside cron (esp PATH) is minimal, so always provide the complete path to any cmd used eg don't just say

date

say

/bin/date

and so on for each cmd.