Editing com.apple.atrun.plist on El Capitan
The file com.apple.atrun.plist is protected by System Integrity Protection.

So boot to the Recovery HD partition (hold down Command + R while restarting), disable SIP and modify the item:


In the menubar open Utilities->Terminal
Enter csrutil disable
Reboot to your main volume and edit the file
Reboot into the Recovery Partition again
Enable SIP again csrutil enable
Reboot to your main volume