This tweak will allow you to send a WhatsApp message at a specific time of your choosing. If you want the message to only be sent after 10 minutes or 10 days, you can do so!
How it works is that when the specified time comes, you'll get a WhatsApp notification allowing you to send the message. So it'll always work whether your device is locked, or even if WhatsApp is closed.
How to use it:
After typing out your message, tap and hold the send button or choose "Schedule message" from the action menu (where you send images), and a date picker will pop up allowing you to choose your desired time.